I don't know that it's a DB design flaw if we're talking about federation messages to other instances inboxes (which created rows of that magnitude for updates does sound like federation messages outbound to me). Those need to be added somewhere. On kbin, if installed using the instructions as-is, we're using rabbitmq (but there is an option to write to db). But failures do end up hitting sql still and rabbit is still storing this on the drive. So unless you have a dedicated separate rabbitmq server it makes little difference in terms of hits to storage.

It's hard to avoid storing them somewhere, you need to be able to know when they've been sent or if there are temporary errors store them until they can be sent. There needs to be a way to recover from a crash/reboot/restart of services and handle other instances being offline for a short time.

EDIT: Just read the issue (it's linked a few comments down) it actually looks like a weird pgsql reaction to a trigger. Not based on the number of connected instances like I thought.