Many people would find it inconvenient to find another device to take a picture with. And that manual picture would then have baked-in evidence that it was taken surreptitiously.
Locks keep honest people honest, as long as the app is up front about it and lets users make the choice, I don’t mind.
Nobody deliberately opted in to random information hiding in a more general purpose app, or watermarking, or DRM.
When I’m in Safari, taking a screenshot can turn into a totally separate operation (full page scrolling screenshot) and that’s a good feature.
Others complain about Google Maps showing a popup telling the user about location sharing upon screenshot - I think that makes a lot of sense, considering it doesn’t show up in the screenshot and dismisses easily.
I’m very harsh on user hostile choices, in fact I spend a lot of time ranting and raving about it, but I just don’t see that here.
Offering something better can be valuable (sometimes) but don't alter the screenshot itself.