And not including post body information beyond anything except the article has always been a strictly reddit based thing.

This is Lemmy and Federated so I'm against that hard.

The entire point of the fediverse and Lemmy is that different instances and the communities within them can set things up as they see fit. And I think being so militantly against something simply because people on Reddit did it that way is silly. There's no reason to dismiss an idea simply because it was done that way on Reddit before. Just let Lemmy be its own thing without worrying about how Reddit does or doesn't do things.

I see no real need why they need to be separated. The difference is negligible to my browsing experience. It does end up making OP need to do one more post though.

The suggestion is based on the opinion that the content of posts on this community should be limited to objective information directly from/about what is being posted. Opinions, discussions, arguments, etc have their place in comments section, that way people can respond to those individual comments. Having the post itself be a combination of an article and a random musing from the OP means top level comments will be a mish-mash of responses to the article and responses to the OP's post body comment. By requiring the OP to post their comments in the comments section, we ensure that all top-level comments are responses to the linked article itself.

If we didn't want OP to have an opinion on something posted, then what's the difference between simply not letting them comment then?

Nobody is saying they don't want OP to have an opinion or be able to comment, it's a simple suggestion to separate the OP's comments from the post body itself.