It's a fact.
Linemen might not make a single tackle in an entire game and go their whole career without running with the ball. Running Backs might go their entire career without making a tackle Players other than the quarterback are EXPRESSLY FORBIDDEN by the coaching staff to 'pitch' (pass) the ball.
Let's have a look at one of the 'elite' Padball players who I have been repeatedly assured would 'kill any Rugby player' by American fanboys:
Julius Peppers - 9 seasons in the NFL - average number of tackles per game: THREE!
(1) Linemen...I'm assuming here you mean defensive linemen? Of course they barely tackle - their function is to stuff the line of scrimmage and if lucky get a sack/facilitate a sack for another player or tackle the RB.
(2) Offensive linemen don't make tackles - they block.
(3) Running backs not making tackles...Yeah, that'd be because they are not supposed to tackle. They run and catch. And block.
(4) A 'picth' is not a pass.
(5) Nobody but a QB can pass the ball, eh? So explain Wild Cats. Explain fake punt/fake field goal pass plays.
Julius Peppers, a defensive end, is your example? So because he doesn't make many tackles he's useless? By your logic Nathan Hindmarsh is the greatest league player of all time and, as much as I adore the guy, we all know that is not the case.
I'm not sure Peppers would "kill" league players, but the guy is a monster athlete. He's tall, he's huge, he's mobile, he's quick, he's strong and despite what you claim he's fit. Your average NFL defensive tackle or offensive lineman are great athletes. As are NRL players.