freeCodeCamp /news is a tech publication, and by design, you cannot cross-post. This makes sense since you're requesting to publish a high-quality article that will be edited and optimized, and publication will give you massive reach. This is not bad, but just simply how their platform works and its why it makes it worth submitting for the publication.
DEV can act as a blog, but it really should be thought of as a forum. If you cross-post to DEV, DEV might respect the canonical URL, but they have a "/comments" page generated off of each article that does not include the article's canonical URL, and this page can rank higher than your article.
There is also the uncertainty of DEV's canonical URL being correctly implemented. They use a service worker to update the heading, and I am uncertain if google read the original or the cached header, since google can execute javascript. I have an open ticket on forem/forem and watching it to see if someone else can tell if its working or not.
That being said, DEV is a forum, and its "/comments" page is not bad behaviour when you think of it as a forum. However, if your cross-post there that could impact your organic reach. So I do not cross-post my long-form articles on DEV but I focus on watercooler content, which is how the intent of forums.