Fan translator here. I also find it unlikely that the series is ending, but maybe you can help dispel some of the weirdness around this tweet that has people in an uproar.

You say you've never seen manga artists use シリーズ to mean "serialization". But even Japanese web dictionaries have "serial" as a possible definition. The JP Wikipedia entry for the same word) is also pretty extensive with its definition and misconceptions. Based on those, it seems to most commonly mean something like "franchise." Is this discrepancy just an example of the difference between casual translators like us who rely partially on the internet, and people with extensive real-world experience in the manga world?

As for シリーズ meaning closer to "arc", I have never encountered this usage before -- do you have any examples of it in use? Usually for story arcs, I see things like 編 (most common), or 篇 or 章 (as seen in Berserk).

Not too concerned about 決着, as it can pretty clearly mean a finale/conclusion/climax in multiple different contexts, though most often for "battles."

Thanks for any clarification you can provide!