The one time I tried reaching it, I got blown by the explosion just before reaching it. If so, what does it say?
What do you mean "reachable"? If you can see it, you just need to hit use (E) to activate it, the range is virtually unlimited, and I don't even think it's hindered by line-of-sight.
pressing Tab will teleport you to the next commentary bubble, and if you miss one it'll cycle through them all again. Not sure if that'll solve the problem of getting caught in the explosion, though.
This can most likely be reached with
sv_cheats 1; noclip, however, don't forget to turn off those cheats by using
noclip; sv_cheats 0 if you still need the achievement for finishing Portal 2.