De nuevo, sí, has borrado todas las instantáneas locales. Pero está claro que tienes una unidad externa de Time Machine. Y las instantáneas de Time Machine (locales o remotas) imponen ciertos requisitos a la unidad de origen. Desde man tmutil :

BACKUP STRUCTURE
     Throughout this manual, specific language is used to describe particular
     "realms" associated with Time Machine backups. It is important to under-
     stand this terminology to make effective use of tmutil and its manual.

     backup source
             A volume currently being backed up by Time Machine.

     backup disk
             The HFS+ volume that contains Time Machine backups.

     backup destination
             In the case of a local destination, a synonym for backup disk.
             For network destinations, this is the AFP or SMB share on which
             the backup disk image resides.

     backup disk image (or backup image)
             A sparsebundle that, when mounted, is the backing store for a
             volume that is a backup disk.

     backup store
             The top-level "Backups.backupdb" directory at the root of a
             backup disk. E.g.,

             /Volumes/Chronoton/Backups.backupdb

     machine directory
             A directory inside a backup store that contains all the backups
             for a particular computer. For local destinations, a backup store
             can contain multiple machine directories, all for separate com-
             puters. E.g.,

             /Volumes/Chronoton/Backups.backupdb/thermopylae

--

¿Recuerdas el error?

Por ejemplo, (quizás causado por el uso de APFS Snapshot por parte de Time Machine) ?

Lee esto:

     snapshot
             A directory inside a machine directory that represents a single
             initial or incremental backup of one computer. The word "snap-
             shot", in most contexts, is a generic term and is not to be con-
             fused with a "local Time Machine snapshot", which is simply a
             snapshot stored locally on the computer. E.g.,

             /Volumes/Chronoton/Backups.backupdb/thermopylae/2011-07-03-123456

     snapshot volume
             A directory inside a snapshot that represents a single initial or
             incremental backup of one backup source. E.g.,

             /Volumes/Chronoton/Backups.backupdb/thermopy-
             lae/2011-07-03-123456/Mac HD

VERBS
     Each verb is listed with its description and individual arguments.

     destinationinfo [-X]
             Print information about destinations currently configured for use
             with Time Machine. For each backup destination, the following
             information may be displayed:

             with Time Machine. For each backup destination, the following
             information may be displayed:

                 Name          The volume label as shown in Finder.
                 Kind          Whether the destination is locally attached
                               storage or a network device.
                 URL           In the case of a network destination, the URL
                               used for Time Machine configuration.
                 Mount Point   If the volume is currently mounted, the path in
                               the file system at which it was mounted.
                 ID            The unique identifier for the destination.

             When more than one destination is configured, the most recent
             backup destination will be marked with the > indicator.

             When the -X option is provided, output will be printed in XML
             property list format.

     removedestination identifier
             Remove the destination with the specified unique identifier from
             the Time Machine configuration. Requires root privileges.

             To obtain the unique identifier for a destination, see
             destinationinfo.

A. El error:

Error: -69521: Your APFS Container resize request is below the APFS-system-imposed minimal container size (perhaps caused by APFS Snapshot usage by Time Machine)

B. Su respuesta:

He eliminado todas las instantáneas locales de Time Machine pero sigo obteniendo el mismo resultado.

C. La documentación:

La palabra "instantánea", en la mayoría de los contextos, es un término genérico y es no debe confundirse con una "instantánea local de Time Machine" .

¿Capisce?

Sí, has borrado todas tus instantáneas locales. Pero también tienes otra unidad de copia de seguridad. En el ejemplo que enlazaste, no había tal unidad.

Tienes que eliminar todos los almacenes de copias de seguridad de Time Machine, desactivar todos los destinos, etc. Puedes volver a asociar tus copias de seguridad más tarde. Pero siguiendo esto, deberías estar bien; en el peor de los casos, descifra tu unidad también (la encriptación APFS es complicada y tiene mucha sobrecarga debido al sistema de clave de unidad/clave de usuario).