I don't think that's entirely accurate. In the alpha, the Devil's Lair was a Level 6 strike but there were quite a few times when I was matched up with Level 4 and Level 5 Guardians. Meaning I'm not sure if there is a level cap for ALL of the activities maybe certain ones like Story and Raid? Keep in mind this entirely based on my experience in the alpha and could change entirely in Beta/release.