The implications of this in the article are mainly focused on the ethics, but at least where the copyright is concerned there is no problem with this -- Midjourney Images are not copyrightable, and it doesn't seem like there's any grounds for Midjourney to complain; especially if the images were uploaded by 3rd-party users and not sourced directly by Adobe under a Midjourney license.

From the article:

> Training on AI-generated content probably wouldn’t make Adobe’s Firefly image generator less commercially safe, and the company isn’t required to say what it’s training on as long as it isn’t misleading consumers, said Harvard professor Rebecca Tushnet, who focuses on copyright and advertising law. But training on AI images, such as those created by Midjourney, undermines the idea that Firefly is unique from competing services, she said.

The critique here isn't that Adobe is violating Midjourney's copyright, it's that Firefly's data ultimately is not as ethically sourced as Adobe claims (keeping in mind that different commenters probably have different qualifications of what ethically sourced training data would be).