Shows a massive lack of knowledge, an ability to read the NASA article they link to and a complete misunderstanding of what any of it means or an attempt to show off hoping to bluff the uneducated.
From the NASA article it links to:
Supersonic flight is one of the four speeds of flight. Objects moving at supersonic speeds are going faster than the speed of sound. The speed of sound is about 768 miles per hour at sea level. That is about four times faster than a racecar. Supersonic includes speeds up to five times faster than the speed of sound!
To put it into perspective, while it 'could' be as fast as 5 times the speed of sound, it was more likely if was around twice the speed of sound at altitude. The stratosphere extends from 8-30 miles (42,000-58,000 feet) up so I suspect what they actually did was fly higher, where the air is thinner and the speed of sound is lower (660mph above 45,000 feet) and it is the altitude that is more significant than the speed.
Then again… pretty much all military jets can fly over 45,000 feet and exceed 660mph. Concorde did it twice a day back in the 1970s (Concorde flew at 1500mph, or twice the speed of sound, at 59,000 feet) so that would be a complete non-story intended to sound dramatic, unless they are claiming either to be operating at 58,000 feet (still not unusual) or hypersonic speed (Mach 5, or about 3800mph..) which the SU-34 definitely can't do… but both the altitude and speed were exceeded by the American X-15 back in 1962.
What might be more interesting is the suggestion that the SU-34 was previously incapable of supersonic speed over 45,000 feet… ;-)