You are correct, it was implied that Tom's purposes and concerns were beyond that of middle Earth and he was not to be trusted.

In fact, that reasoning is one of Tolkiens weakest in explaining away how Tom's power is worthless in the fight.

Newbs like to pick on the eagles (which is dumb, it's obvious they would have been shot and/or killed by the ring wraiths/orcs had they tried to fly into Mordor; yet Tom pops up with all this power of the Old World and then he's written off as useless.

Like many fans, I enjoy his character in the books, but it is clear why he's cut out of all the adaptions: he doesn't do enough to further the quest and he's a plot liability