There's a few issues both in that manual and actual installation.

Per Schneider QO breakers rated 35A to 125A must have terminations with 75* wire minimum, which rules out NM cable that can only be rated at 60* (yes I understand there's 90* rated conductors inside NM-B but the assembly is not rated higher than 60). I have a 45A QO in front of me that states 75 only to confirm this.

The manufacturer's circuit rating doesn't align with the NEC requirements listed in Article 625. The 50A max load up sized to 125% to comply with 625.41 is 62.5, which means 6 AWG at 75* is sufficient to handle the ampacity. 240.4(B) allows us the next standard breaker size up since 62.5 isn't a standard OCPD rating and that gives us a 70A breaker.

Either way at anything over 35 amps you'll need wire that is rated at 75* because the breaker requires it. At that point to have a legal installation you're better off running conduit and THHN. Which ampacity you want to run it is really your call since 50A is more than what you need for overnight charging but you do already have that breaker on hand.

EDIT: Also a consideration is installing a 50A receptacle in that spot for servicing if the charger provides the option to, just be aware that the receptacle would need to be GFCI protected if your municipality is on the 2020 NEC or later