First, Welcome to Dakka and back to 40k.
This is indeed an anomaly. GW’s rules are a lot more rigid than they used to be, with “no model, no rules” being the unofficial word of the day. The rules will let you field what’s in the box. Mostly. They are wildly inconsistent. But probably a shock compared to the lawless wilds that was 2nd. The fact that there is a current model without rules is very odd. Sometimes niche or limited models get their rules in a legends line, so official, but not recommended for tournament play.
I suspect that 99% of the people you meet would be fine with you fielding him as a powersword/plasma pistol captain. It’s not like that’s some min/max exploitable combo. If you want to be 100% rules legal, you would need to have him counts-as one of the other options though. The suggestion to call his sword a chapter relic that hits like a powerfist is a good one.
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