It looks like they weren’t the first, if I’m not mistaken that graffiti carved into the stone in the first photo.
Kilroy was here???
Two college friends of mine did exactly the same thing back in 1974. The guards spotted them, yelled a lot, but did not climb up after them. They spent the night (cold and uncomfortable) at the top, which is flat as the pinnacle stones were removed centuries ago, and came back down as it became light enough to see. What these Russians did is no biggie. Thousands of people have pulled that stunt.
When I was in Egypt, I saw graffiti carved into temples, usually just a name and a date — everything from French soldiers in the early 1800s to Englishmen making the grand tour in the 1920s.