This is very reasonable if you have compliance needs or similar. That’s not what this office is saying - it’s saying teams is more secure. This is wrong. The nature of banning private messaging apps is trading security for legibility. If this office is interested in that (which it’s not - it allows Signal), they should say so.
I do have a compliance need, similar to this office i imagine.
Teams is more secure in my opinion.
I as an admin can control who you can/can't talk to, what you can share with them, when you can share it. Correctly configured MS Teams is a pretty secure setup.
On the flipside im not sure i can make someone else's Whatsapp not auto download anything sent to it.... The two apps aren't really comparable unless I've missed an entire 'Whatapps for government/enterprise' business arm.
Microsoft maintains specific secure government versions of Teams that use their own special secure data centers. It's a full parallel extra secure set of infrastructure.