Both planes cruise at around 100 miles per hour and blend in with civilian air traffic, making them difficult to intercept.

And the ruzzkis are not aware that there is some plane coming from Ukrainian airspace travelling 100s of kilometres around their forlorn country? They are that blind on the border to an active battle ground? And why would there be so much civilian air traffic near that battle ground that they can't oversee all air traffic?

Remembers me btw of German bumblehead Matthias Rust who flew a small single engine plane into ruzzia and landed in front of the Kremlin - in the middle of cold war. They simply didn't see him coming until he parked his plane right in front of their door.