monolithic source code can be deployed across multiple servers.

A true monolith is like a true function - it's absolutely useless because everything it does is pure and therefore it can't interact with reality at all.

In less metaphorical terms: you still need to sync external APIs, sync filesystem with the database, deploy to S3 as you don't want to saturate your web workers, or have conflicts in your parallel red-blue deployments.

If your monolith doesn't do anything I had just described then you probably aren't doing anything exciting anyway so it's quite unlikely that you've got a lot to say about software architecture.