This is a strawman argument, but I will assume you made it in good faith given how widely Damore's claims have been misrepresented.
From what I recall in the memo, I'm pretty sure he never said that women were less effective at their tech jobs (fortunately, because there isn't any evidence remotely suggesting that !)
What he did is that, when Google asked for feedback on their mandatory diversity training, he challenged the prevailing view being taught in these classes about why less women get into tech than men (basically: not because of inherent lack of interest but because of societal pressure)
He pointed out that the latest science on "interesting in things vs people" disagrees, and he supported his argument with citations.
In a company that prides itself on making data-based decisions, starting a discussion in this way with a reasonable argument (even if later refuted) ideally shouldn't be a firing offense.
Now, was it politically, humanly, emotionally smart for him to say things in this way without consideration for their impact on coworkers ?
Of course not. But still, I can't help feeling somewhat sympathetic to someone with child-like naiveté who spoke the uncomfortable scientific truth (or at least, a very reasonable theory) that adults don't want to hear.