No, there is no definitve proof of it.

The terrorist told that they worked with someone, who provided them guns and promised a life in luxury. And Ukraine has been constantly working with random groups inside Russia, so it is a possibility, but so is ISIS.

( I question if ISIS would promise a life in luxury, but so do I question the terrorists reliability.)

Also the terrorists appear to be somewhat combat trained if you inspect the footage closer wich supports the ISIS claim.

Probability the truth is somewhere in the middle and both where somewhat involved.

(I suspect SBU wanted someone to burn the theater down and hired people that wanted a bit more than just arson. But Idk.)

I like how everyone nowdays uses “cope cages” and they are still called that.

Cope?