I think the answer is self evident within the article:

I think there is more going on underground than salt-mining. It is a secure warehouse for Ukraine’s military equipment.

It's where they've set up their logistics to protect against scenarios like what they're encountering.

However, is it possible that Ukraine has stored evidence of its bioweapons program in the mines?

What a dumb speculation

That reminds me that Russia has been really quiet about this piece of propaganda for a long time. I guess it didn't stick after all.