Besides the fact that semicolon-separated values are well-established, using this tagging also improves likelihood of redundant wikidata elements being reviewed and fixed.

Some wikidata enthusiasts might regularly check all unusual wikidata tags in their area of interest but they wouldn't watch out for a undocumented wikidata:1 tag.

Btw in this specific case, wikidata entries have already been merged (thanks to seav) and wikidata tag in OSM is now updated too.