If Ukrainian drone teams and others are using it to publish their footage, in what sense is it not being used by the Ukrainian military?

In the sense that it's not used for military communications.

I seriously doubt you will be able to prove it.

Ukrainian military bloggers use it, their public persons use it and while there is probably an an order that dictates not to use it - I really doubt it is being followed completely considering the vast majority of Ukrainian military are yesterdays conscripts and not professional soldiers. Some of them drink, some of them sell their equipment, some of them gamble.

I really doubt they don't use telegram. Maybe it is not systemic but neither this is for Russia.

Our military's comms are over Matrix/Element, Whatsapp, Signal (in the descending order of significance.) Officers are trained to avoid Telegram just like the other known-to-be-compromised communication channels.

Nobody who's got a secret to keep would use it.