How is that hard to judge? Sans serif have largely the same overall design characteristics, but you can clearly see the differences in the design of individual characters. Font design nowadays is about the fine details and it’s clear from the outset that Apple has had a different starting point.
It's normal that some people will think that they are very similar. The differences only start getting obvious when the letters are bigger.
But the question is whether Apple copied Google. That’s not just a matter of opinion and as such it requires a closer look and then it becomes clear that they are not the same. Moreover, San Francisco is just the collective name for a whole list of fonts that include both the cuts for the Watch and iOS/OS X (with different shapes), the two optical sizes for each (with varying spacing and kerning) and of course the widths and italics.