You should also keep in mind that not everyone uses ezine for SEO. Those accounts you saw with thousands of articles could easily just be using the platform for other purposes.
I don’t think there is really any added benefit to posting thousands of article all to one site vs spreading them out over several submission sites. I suppose one plus could be a diverse backlink profile but that’s about it. Just make sure that you are posting quality stuff or it will just get taken down. Also, people tend to not read through or follow links from poorly constructed content.
There are a lot of good answers in this thread,