I think that is a bit of "letting the student do the math"
I think that is a bit of "different writers writing different movies set in the same universe".
But, in universe, you have Xavier, showing the effective new guy, Logan, the ropes of being a proper mutant and then showing him the use of Cerebro. To which Logan quips "why don't you use it to find Magneto?"
"He seems to have found a way to shield himself..." is a nice way of putting what he really should have said.