They're right in that tanks are more likely to be operational later into games. It's now very difficult to one shot vehicles with anything other than a melta (same for AP2), and you need a penetrating hit to do any damage at all unless you wipe out all the hull points.
There's also a lot more pressure on builds to take plasma to counter AP2, and flamers for Overwatch. Then there's flyers and flyer defense. All of which means less room for the standard max mech and melta of 5th. Just building to kill tanks is going to hurt you in other areas.