Me too... this is actually the reason I can't use Safari as my default browser, despite wanting to. The inadvertent gesture keeps showing all tabs, which is so interruptive and annoying as to drive me away.
I need pinch-zoom in too many other apps to disable it globally. Interestingly, I don't find myself constantly triggering it in those other apps, or if I do, the consequences are so trivial that I don't notice it and it doesn't interrupt by train of though or require a complete cognition reset in the way switching to tabs does.
Safari's interpretation of "zoom out" to include switching from re-scaling content in the current window to going up to tab view is a non-standard one that I'm afraid is just too clever by half. It's a cool idea, but since it's different from all other apps treatment, should be an app-specific preference that can be disabled.
I hope Apple makes this an app option override soon. I grew tired of Firefox's constant craving for additional memory, and would like to get away from the sense that Chrome tracks me in ways that the best blockers can't really seem to stop. But I'm accepting both of those over this aggravating window behavior.