I dunno. It passes the sniff test - Josh Allen is the highest rated at the combine and has what I think is the biggest arm.
Quarterback Ball Velocity at NFL Combine 2008-2020 (ourlads.com)
Okay sure - I know what you are thinking maybe guys aren't really trying. But when they do try those are the kind of numbers they produce that is in the normal range of throws..
Check out Mahommes trying to throw it fast..
How Fast Does Patrick Mahomes Throw A Ball?! | NFL Total Access | NFL Highlights HD - YouTube
Ryan Mallet has a huge arm - maybe he wasn't the most accurate but he could throw it pretty far.. And again combine numbers seem to reflect that..
It's too bad Mac wasn't at the combine because as with his run speed - I think his arm would be better then people think.. I'd wager that Mac Jones arm would be as scouted - NFL average so like 56 in velocity... I think they do track this with the advanced stats - so would be very curious to know what his fastest pass was this year..
Another guy who claimed Mac Jones has a weak arm is Rex Ryan.. He called it a pea shooter. The scouts didn't claim that though - just said it was not elite. AKA among NFL QBs it would be average. If you really want to think about this you have to ask among NFL starters who has a weaker arm?
Tua? Garrapollo? Tannehill? Darnold? When you think of it like that - its pretty likely his arm is NFL average. Not that this dooms him or anything but if OP really wants to know where this idea comes from - it comes from scouts and rex ryan.