Just completely not true.

Let's look at the DL's main job--stopping the run. Best judged by yards/rush given up.

2017 (Tuiaki under Sitake) 3.69
2016 (Kafusi et al under Sitake) 3.66
2015 (Kafusi under Bronco) 3.28
2014 (Kafusi under Bronco) 3.86
2013 (Kafusi under Bronco) 2.74
2012 (Kafusi under Bronco) 3.66
2011 (Kafusi under Bronco) 3.9
2010 (Kafusi under Bronco) 3.9
2009 (Kafusi under Bronco) 3.4
2008 Kafusi under Bronco) 3.9
2007 (Kafusi under Bronco) 3.0
2006 (Kafusi under Bronco) 3.6

10Y Kaufusi under Bronco 3.52

So if Kaufusi underperformed (which he didn't because BYU's avg rank on that stat was about 30th) and Tuiaki came in and made it worse, Tuiaki needs to be fired. But then, so does Sitake. And so does Holmoe. I'd look for that next year. At any other school it would have been this year.