This absolutely was the case for a long time. It was the cultural center of the internet where nearly all memes sprang from or gained traction and context before leaving orbit for the greater internet.

That has not been the case for years though. I'd say it shifted to twitter as things shifted to inseparably political on almost all of 4chan maybe 6-8 years back and then shifted away from twitter a while after elon bought it and a lot of people started to bail. and I honestly don't know where exactly it's shifted to now, but I'd have to guess tiktok and similar new platforms.

But regardless I do think 4chan has lost nearly all of it's cultural influence, but still maintains it's notoriety.