You've lost the context of the Advice Memo. The NLRB isn't saying Damore himself violated EEO laws by writing the memo. The NLRB didn't fire Damore; Google did. The NLRB is saying that although the NLRA protects concerted action to improve working conditions, those protections do not extend to action that might discriminate against protected classes, and, crucially, that because employers are required by state and federal law to comply with EEO laws, the NLRB will tend not to second-guess them about how they do that.

The important thing to remember is that in most of the US, there's a presumption that employers can fire you for any reason. Employment is at-will. Damore was appealing to a specific exception to that rule.