In the original VCOH, 'engine damage' happened regardless of armor.
Usually one sticky is enough to engine damage a King Tiger.
Making AT nades weak against heavy armor in this game was a design mistake.
On the other hand, in CoH1 the target needed to be in range for the entire animation, or your guys would just walk back toward the vehicle and try again (often taking casualties). Aside from that, PE's AT Grenade didn't give engine damage and it had a fixed trajectory, if the vehicle moved while it was in the air, it hit the ground. The Panzerfaust also didn't give engine damage, and there was a good chance it would even miss.
CoH2's infantry AT abilities are significantly stronger because it almost always does engine damage, and you only need to be in the range for the animation to play, after that it's homing and has unlimited range.