I don't know if that's really true anymore. Or, maybe more accurately, I think we're in between when that was true in the pre-social media age and when it'll be true again after the social web breathes its last. There are just too many ways that megacorps, bad actors, and foreign agents can manipulate offline activity with online action.

Some of those things are manipulating small groups into large-scale action (see: Qanon), but misinformation and meme warfare also have a measurable effect on election, direct action, etc. Not to mention that local organization is best done online, and that has a very real effect.

Now, is offline action more important than online action? Absolutely, and if you're saying that being an "online leftist" (as an identity) is a distraction, I think I agree with you. But online action is more than just a distraction, and to ignore it is probably counterproductive.