what he's saying is you're replacing a well made stock intake that seals off under hood heat and pulls air from a cooler location, the fender. with a aftermarket air intake while having a slightly better flowing air filters that is much worse at filtering dirt, almost all of them do not seal off the air from under the hood. so while marketing calls them cold air intakes, they are actually hot air intakes, decreasing performance on a daily driver because intake Temps go up and ecm pulls timing based on intake air Temps.
all the dyno runs you see arw done on a cold engine with the hood open and a fan blowing on the vehicle. in the real world you most likely get a slower truck, worse filtering with a side effect of intake noise that most people think means it's faster.
it is possible to improve on the oem design, but most aftermarket setups do not. and then you gotta factory in that they build the airbox for a 6.2L making 400hp and it's running on a 300hp 5.3. there's probably not a lot of hp left on the table, even if you pull the filter off and ran open intake.