Thanks for bringing this to Meta.
In case of Cross Posts that I run across, I like to leave a comment for the OP explaining that cross-posting is heavily discouraged. They often just don't know the policy.
In these cases I sometimes flag the post immediately, but I often just star the post, and wait a day or so, to see if the OP can work it out themselves. In most cases they do, and often express their gratitude on having it explained to them how things generally work. If they don't work it out, then I will raise a flag.
So I would highly encourage you, if you have time, to leave a comment on the crosspost explaining the policy, to help teach the OP to fish. But in either case it is OK to raise a moderator flag, since the moderators are here to handle the odd situation such as these.
I would like to discuss one specific point from your post...
I flagged it as a cross-post: it's never ok to cross-post on both sites, see When is it okay to cross-post to on Data Science from Cross Validated?
While Cross posting is heavily discouraged, it is not absolutely forbidden. Even in your linked answer, (from ChrisF, who is a moderator on multiple sites,) you will note that there can be exceptions that prove the rules. Additionally, ChrisF starts his post with: "Cross posting is a bad idea. It's rare that exactly the same question is on topic on both sites". Both Bad Idea and Rare imply to me that the practice is not absolutely forbidden.
Another relevant quote, this time from the Director of Community Development Robert Cartaino:
In the edge case where the question is appropriate on more than one site, leave it on both sites and let the users of each community benefit from the information.
I am not condoning cross-posting, just trying to note that there is no absolute here.