A counterargument to that is that the flamenade doubles as a very fast, powerful cover denier. If OKW doesn't have it then they struggle with early buildings, if they do they can use it to swing the first fight anywhere on the map.
Locking it behind the truck means OKW doesn't have that flamenade for the first fight and that they call the truck in as soon as possible, slowing their T0 by 100 manpower.
Leaving it be and weakening OKW T0 in other ways means you're balancing around OKW pulling off an impactful flame grenade in the first fight, making it more important to that initial engagement, not less.
Flame nade very really has any impact on the opening few engagements unless its a unit closing in on a volk, pretty rare a closing in squad beats a volk at vet 0 with or without a flame nade.
It's almost always a flanking sturm with a bunch of volks sat at long range that kick starts a snowball. Moving sturms damage or slowing down how fast OKW cap early game stops their snowball, it doesnt make their early game bad it just puts it in line with every other faction in the game