I think girders simply shouldn't exist in BnG. Because ANY girder you place will at least partially block some shot for the opponent, so there's always an ambiguous rule about how much you can enable your shot / disable your opponent's shot. Not only that, you'd have to have some dumb rule about only being able to use a girder when you have no other grenade shot available.
In this case there wasn't any reason to use a big girder over a small one, but there was no significant shot blocking going on, the worm was still quite open. The gap that was blocked really wasn't an optimal place for a shot, and I don't think I'd even consider this cheap since there's no reason I would be aiming at that gap.
But additionally BnG needs to take steps to start disregarding 'cheapness', because although the concept helps maintain a higher skill standard, it can only work in a league like a2b, it should be utterly disregarded otherwise.