Yes, that’s what it is, and it is a more or less chronic problem for me on the bike. A little training for it should help. Pushups are good, but I can’t do them now, but any kind of gentle strengthening should help.
Not enough to be a serious problem, just that it's the current weak link. I'm happier having it be the arms than my lower back or something else
Interesting that Ian Smith did a ride at 11 PM that was almost exactly the same length, climb, and time, but for him it was an easy ride where he felt tired and sluggish and it was logged as level 1.