Thank you Mr. Doyle, but you're still trying to impose the principles of physics on a universe where we know either none exist, or they're so perverted from our own that trying to derive anything meaningful from the scant data we have available to us is an ultimately fruitless endeavor. Personally, I prefer Socrates to Holmes: the only true wisdom is in knowing you know nothing. Come to think of it, Occam might also have something to say about the inevitability of your conclusions.
Actually Occam's Razor says that the simplest theory that fits the observed phenomena is generally the correct one.
We observe that they obey many of the same physical laws we do, because they still have gravity, chemical interactions, etc. We also know that the air that surrounds their worlds has resistance, because otherwise they wouldn't be able to breathe, which they clearly do. This means that their ships would also encounter it when they travel through the atmosphere. This leaves two choices: either their ships are made from materials that can withstand ungodly high temperatures, or they've developed a shield that can project around the ship. Since none of the ships seen to exit an atmosphere have any visibly glowing surfaces in space, where heat radiation is a huge problem and luminously glowing metals would stay glowing for long periods of time, the only other conclusion is they must have a shield.

