Seems to be using the Observer's Level instead of the Caster's Level to judge if the spell is low-rank.

https://discord.com/channels/868914725784797224/878370187601150003/888993283978715136

A level 30 Warlock should be OK to use the spell that is listed there.

https://tbc.wowhead.com/spell=19835/blessing-of-might#see-also-ability

Confirmed it's looking at the Observer's Level not the Caster's Level.