It's also a very old technique to proxy attacks through countries without extradition treaties.
There are numerous incentives that, to me, make it not only reasonable but extraordinarily likely.
To me all this seems par for the course. There's nothing unusual about any of it. It's what you would expect. It's basically like distributed stochastic terrorism, indirectly/loosely driven by a more capable state actor with specific intent to establish deniability.
Not even plausible deniability. Just some convenient way to say 'Nyet! And we are VERY OFFENDED that you would even suggest such a thing!'.
Just the fortunes of war, really.