1) "Not entirely obvious! A great place for a dogwhistle!" I'm obviously not suggesting that whatever graphic designer intentionally slipped a Nazi symbol into Slack's new logo. But, much like the arrow in FedEx, once you see it it's hard to unsee.

2) Most people can't tell you which way the outer parts of a Nazi swastika go and which way non-Nazi swastikas go. That's what I meant about sense of rotation in my comment above. Moreover, most people can't immediately reconstruct a logo to begin with, though they can recognize one. But something like this https://imgur.com/a/CZWtXez passes at first glance and is much more swastika-y.