Are the spam DMs advertisements or more generally something linked to a product or service? I wouldn't be surprised if X is more lenient towards bots that pay them for adverts.

Most of what I get seem to be advertisements or automated messages if you follow large(r) accounts.

One of the most interesting things that I've noticed is these advertisements will be triggered if you follow accounts that are positioned as influencers. I followed one out of curiosity and received a DM from that account advertising some cryptocurrency service.

It's a good way to filter out and block accounts that have almost certainly not grown organically.