I guess we can follow AI translation progress by using "kissanristiäiset" and see if it comes out as "cross-dressing" or something else. Watch this space... (Google is already more correct, so dunno where the Twitter folks even got that...)
Helsinki is like 100 miles from Turku where Rantanen spends his summers, for cultural context it might be important (or not) to know that there's this half-jesting thing where people from Turku still think their city is somehow specially important for having been the capital under Swedish rule over 200 years ago, so for a Turku native being in Helsinki might by default mean less focus in truly important things, like being in Turku...
Love it.
Anyhow, my take on this just based on being Finnish and kinda knowing the culture:
1. Rantanen doesn't seem to be joking, he seems serious (somebody posted the Valtteri Bottas F* You thing, seems like similar feelings here).
2. Artturi has nothing to do with this and the relationship between Artturi and Rantanen is going to be just fine, they'll laugh it off the next time they meet; everybody knows Artturi's dad is what he is and Artturi knows it better than anybody (and as others have said, probably gives Mikko a high five afterwards, lol). I think all the Finns here can agree that this part is true no matter what, the situation is much different with these two players than with some other random players.
3. Once he calms down a bit, Rantanen will brush it off and it's going to be fine. He'll give some s*it to Lehky's dad on the phone or the next time they meet but it's not going to affect the long-term relationships of anybody. I bet Rantanen will be training with Lehky's dad in the summer again.
4. It could all still be a joke, but usually a Finnish guy joking has at least some discernible tongue in cheek, whether it shows up in the face or just a little in the voice or the way things are said, but here saying "shit's going to come back to you" just doesn't seem like a humorous jab to me, more like an emotional "I showed ya". Just goes to show that even the Finnish players are human beings and not machines, and things said can sometimes get under their skin. It's a nothingburger with fries on the side and once those fries are gone, everything's fine again.