You're thinking too logically here. This is about politics and how it looks from the outside. For someone not in the know about composer and provider and alike it now looks like:
1. There is a package wordpress/wordpress
2. wp-downloader has something to do with it.
I can already see the headline "WordPress now on Packagist as wordpress/wordpress!!!11"
Is that reasonable and fact-based? No, of course not. But still my gut says somebody will not like the looks of that.