The thing that ticks me off so much is that they didn’t need the moth at all. They could have just been spotted by the eagles like in the book. Spotting things from the air across great distances is what eagles do.
I guess one could argue it would be lazy storytelling for the eagles to spot Gandalf and the dwarves at just the right moment to rescue them, but it still wouldn’t be any more of a coincidence than Gandalf coming across exactly the right species of enchantable moth at just the right time to send out his SOS.
