Sorry, I misunderstood--
I'd thought that that part of the assertion had been self-evident. Yes, obviously on the one hand you have "game mechanics", but purely fluffwise we know the following:
1. They began with small, light, infrequent attacks using only their weakest troops. They slowly scaled up the threat level and intensity of their attacks over time.
2. Despite having psionic mind control which is demonstrably capable of functionally puppeteering human governments, they establish such holds on macroscopic power infrequently, slowly, and only in areas where XCOM repeatedly fails.
3. There aren't massive swaths of land completely overrun by Chryssalids, despite a certain mission demonstrating that it would be laughably easy to seed a few dozen locations with nascent hives.
4. Despite having a fleet apparently measured in scores if not hundreds of ships, the aliens launch them piecemeal, deploying only a few at a time, and scaling the threat similarly to their ground units.
5. As above, Meld Canisters. More specifically, their distribution. I think it's fair to argue that they're distributed intentionally to get Meld into human hands as part of the uplift process. Remember how Meld works? You get more when you've been doing poorly on missions and less when you've been doing well.
6. With the size of their fleet and the quality of their shipboard weapons, the aliens are easily capable of Independence Daying a few dozen major cities, which would undoubtedly make it easier to subdue humanity. They didn't.
7. Connected to 4 & 6: Early on, you've got four interceptors and shit weapons. If they just wanted to conquer Earth, why didn't they launch a few dozen ships every time you sent out an interceptor? It would be painfully simple to utterly shut down XCOM's early air game even with half a dozen fighters, never mind a battleship or ten.
Do I need to go on?