Not necessarily true. Every culture and species will have it's own perceptions and blindspots. Weapons and tactics are always developed in response to the tactics and capabilities of your enemies. The alien technologies would have been designed to respond to whatever threats exist to them 'back home'. And a lot of technological improvement come from the combining of cultures and mindsets - culture A invents something that eventually finds it's way to culture B who then see a new, unique use for the technology and that use makes it way back to culture A who incorporate it and improve upon it for their use.
The easiest way to conquer Earth would be with bio-weapons. Create a virus deadly to humans - or raid the CDC in Atlanta, The Pastuer Institute in France, or others - and steal some strands of Ebola virus (among others) and genetically modify it to be more deadly and get rid of the virii's weakness to UV and chemicals. Then use the Thin Men to introduce it into our society, back off, and then invade during the chaotic aftermath of having another Black Plague ravage the world.
The aliens don't do this in XCOM though. Good thing - as it would be a very different game. It could be they've never considered the idea of 'weapons of mass destruction'. Or that something in their own history/culture/religion makes the use of such weapons so abhorrent they don't even use them against the enemy. It could even be one of the reasons we're invaded... after hoping (and maybe nudging) humanity to destroy itself during the cold war and failing, the aliens have realized that invading might be the only way to keep the race of violent, self-destructive monkeys from spreading into the galaxy and creating havoc.
Whatever late game Terran tech exists will be heavily influenced by alien technology but it will be married to human thought processes and experience. The weapons we develop will be in direct response to the current alien threat. Also, say you understand how plasma works and how to build plasma weapons... no one is going to say "Hey! I've got an idea for a gun that's *almost* as good as a plasma rifle!" Nope, the "eureka" moments will be how to take aspects of the plasma technology and make it even better.
oh.. for those of you who are interested in current real technology, check this out:
http://www.youtube.com/watch?v=TXECU3YKMfI
It's about a weapon system called Metal Storm. The first 2 minutes explain current gun technology and the information about Metal Storm starts at about 1:50. At 5 minutes and something mentions the ability to fire 40mm grenades at 250,000 rounds per minute.
