Interesting, the spreadsheet has it listed at 21". Did you measure without the padding? I would say that should only make .5" difference, though.
The thing is, I think the reason we tend to "need" super-tall belt guide HBBs is for cars without headrests; most of the time if a child has a 21"+ torso it's appropriate for them to be in a NBB. Generally speaking, then, if they need a HBB it's to provide them with head support. So even if the Apex IS taller than the Frontier, it's of limited value, know what I mean?
BTW, was that in the top locked position? That headrest will slide up until it comes out the top, so although you may be able to pull it up to 24", it's possible that that's not a real "position." I just got rid of mine, so I don't know that I can double-check.
I did love the fact that I could fit my non-skinny booty into the seat, though.
Most of my dck are chunky (they're either chunky or peanuts, lol!) so it was great for that set.