They can challenge foul play, which didn't even happen, but the bunker isn't allowed to look at discretionary calls.
How many times have there been markers blatantly not square or people well offside on the goal line, but a team never challenges it or that call is never made on a challenge even if it happens... because if they were allowed to they'd get the call all the time and captains would be challenging them all the time.
The NRL now need to come out and confirm one way or another... if they want to suddenly open it up for markers not square, offside, etc. then it will be a complete shit show... they'll probably just ignore it and hope that it goes away... but teams like the storm would've seen that today and taken notes that marker not square is on the menu.
Also the original rules around the captains challenge included that they couldn't challenge for foul play... because there'd be too many times where a touch is found to be slightly high and the bunker would have no choice but to award the penalty... just like DWZ's today... but NRL decided to ignore that rule in the game we lost to rorters when Teddy challenge on a whim and the bunker found a finger on Tupouniua's chin and gave them a kick to win the game... and now it has seemingly disappeared off the face of the earth.
Although every now and then a bunker will throw out a "the contact is high, but it's incidental"... who even knows what that means, but they've decided to give themselves a get out clause... but only if they decide they want to use it, because it doesn't come out all the time.