Their explanation of the 5M limit is because of inode limitations when they materialize the content via the companion client?
Perhaps the Windows exposed API assumed somewhere around 5m objects, and they care about the compatibility greatly.
It could be a number based on usage. Say 99.9% of the users use less than 5M files. So my pruning 0.1% of the user base they are getting rid of half the number of files, saving X millions of dollars in cost.
Nobody wants to cater to the difficult users that are doing stuff like this.