What is the point instead of having them in two separate repos?

Any full stack change will be represented by one PR that changes from pre change to post change. Two repos would introduce a new possible state where one has the change applied and the other doesn't.

And later you add the iOS and Android clients too. Will those go into the same repo? Better to keep server and clients apart, especially if release schedules are different.

Sure if the release schedules are different then have them in separate repos so things like tagging makes sense. But often people work with a single release schedule. There's just so many variables that go into these decisions that the thread here is bonkers.

Smart people can work through problems to get the job done. Monorepo vs polyrepo won't stop people from moving forward.