There are a lot of gotchas with using DMS (which seems to use pglogical under the hood). Since it’s not hardware-level replication, you can run into issues with large rows/columns/tables and it doesn’t really handle foreign keys. It may not handle some special data types at all. You also need to update the sequences after the migration or you’ll get errors about duplicate primary keys. You can also have issues if you don’t have proper primary keys, because it doesn’t always copy the entire row at once.
If the databases are within the same AWS account, it’s likely easier to use hardware-level replication with global database or snapshots to do migrations if you’re ok with 4-5 mins of downtime.
There are many options available with PostgreSQL you could also do a physical full backup + WAL level replication to keep key AND a get low downtime.
What might have oriented theirs choice is that they wanted to upgrade from major version 11 to 15 during the migration process. This is only available using logical replication. Otherwise you'd have to chain upgrade process of each major version (and possibly OS because 11 is EOL on some arch) and this is nor trivial nor quick.
If the databases are within the same AWS account, it’s likely easier to use hardware-level replication with global database or snapshots to do migrations if you’re ok with 4-5 mins of downtime.