I think you'll find the ban covers more than just work phones
The problem with mobile devices surfaced a year or two ago when it was discovered that phones and fitness trackers were able to provide valuable intelligence about the disposition and activities of service personnel. This goes way beyond a particular application but it shows that our leadership is all at sea when trying to distinguish real threats from science fiction. Judging by what TicTok does its not going to give any additional information away that's not already readily available from the mobile device (i.e. why bother with a secret program of spying when you can just buy everything you need cut price as 'analytics'.) The fact it was written by Chinese people -- probably -- doesn't make it any different from any one of the hundreds of other applications competing for our attention (and, anyway, if you fear Chinese people working on software then you've never worked in California.....).