They allegedly remove posts/comments about lemmy? And even if they don't, I feel like it could have the opposite effect. People would see those posts just like ads/promotion/spam. Which would give lemmy a bad rep. Unless something big happens, like some big community switching to lemmy, or someone with a big following promotes lemmy, it will hardly see a big spike in user count.

The only way is to passively "advertise" it. Maybe add the link to your lemmy account in your reddits about you section, if you are making OC add your lemmy handle there as well...

And the last way, which is most likely the best way to do it, is to post good content on lemmy, keep communities alive. And people will eventually join.