I'm not coinbase, so I can only speculate as to their meaning. While it's possible to define "political" so abstractly that it becomes all-encompassing, it's pretty clear to me that they probably mean to prohibit political debates that are unrelated (or only very tenuously related) to their core mission.
"We should hire a more diverse workforce because diversity improves our collective judgment, thereby furthering our mission" is on its face a fine subject for discussion provided you're evaluating the claim on its merits and not sneaking in overtly political implications like "we have a moral responsibility to hire diverse candidates". One could envision a fruitful, dispassionate discussion about how much return on investment the company could expect from a diversity initiative, how that ROI stacks up against other opportunities, how to measure the ROI and make sure the initiative is meeting expectations, whether or not it runs afoul of anti-discrimination law, what kind of diversity (race, gender, viewpoint, etc) is most likely to yield the highest ROI, etc.
Of course, it's prudent to stay away from these kinds of charged topics altogether because it's very likely that you have employees that will take this as an opportunity to pollute a potentially productive conversation with their overtly political opinions.
That some things are easily abused by bad-faith employees doesn't mean that we should let bad faith employees have free reign.