true, but you want to make sure you don't alter the structural integrity of the item
in the case of an airbox it probably doesn't matter, butif it were something more important, say a radiator, you don't want to make it thinner in one area because you melted it, even if it makes a nice tight seal.
of couse actually getting it properly done by a plastic welder is different, but DIY with a soldering iron doesn't sound like a sane idea from my end