I phoned Finnair customer services and they were intially adamant that on a superlight fare there is no free carry-on bag allowance regardless of any frequent flyer status (they didn't suggest even Finnair status got any carry-on allowance), only a checked baggage allowance.

I pointed out the article from 2023 at the launch of the superlight fares and asked had it changed since then or was the article wrong. They went away and came back and said I would get a free carry-on bag due to my (non-Finnair) oneworld status. I asked if it would say this on the boarding pass, but they just said they would know at the airport due to the oneworld status.

So quite probably it would be ok at the airport, but to be honest I wasn't confident they wouldn't just say anything to get rid of the call knowing it would be someone else's problem.

I really don't want to have an argument at the gate, so I just chickened out and paid the carry-on bag fee. At least it only turned out to be £11.60 per person rather than the per flight I was expecting.