It seems like they recommend splitting out the database at the start because using a managed service is much easier than properly managing your own production database.
I can't speak for high performance/near or realtime system people but I would not trust a managed database service for those needs. My experience is that the managed offerings lag behind upstream versions substantially and usually are economical because they are multitenant. So you have a bit less predictability in io wait / cpu queue, lose host kernel level tunings (page sizes or hugepages, share memory allocation, etc), and - not naming names - some managed db services are so behind they lack critical query planner profiling features. That's not even going into application workload specific tuning for various nosql stores. This is a nice article but its audience is people that haven't scaled up a system but are trying to cope with success. It's not great generalized scaling advice.
Those all sound like concerns that probably aren't your top priority when you have 10 users unless your product is performance critical, and then none of this guidance applies anyway. When you have 10 users, your database concerns are more likely to be: Is my database up? Is my database secured? Do I have automated backups? Am I otherwise at risk of losing data? Is the database fast enough for now to not be a blocker to my business? All of which are concerns that tend to be well covered by managed database services.
All I will say is that our latency getting a business entity in or out of a SQLite database (running on top of NVMe flash) is on the order of tens to hundreds of microseconds. There will never be a hosted/cloud offering that can even remotely approach this without installing some "bring the cloud to you" appliance in your datacenter.
That said, it's not a fair comparison and I wouldn't want to run a big service on a single sqlite/nvme setup for more reasons than are worth mentioning, but not prematurely optimizing can take you really far - scale and money - with good design.
