Base set 2, first set (outside Fossil) to feature the cosmos holo was released February 24, 2000. The Galaxy Pichu card in the referenced thread was released (in English at least) July 8, 2000 during the 3rd movie release. The copyright date on the Italian cards is 1999-2000 so it was most likely printed during 2000s (cosmos holo would suggest that too). This must have all happened around the Galaxy → Cosmos transition.
This could be a case of just WOTC using whatever they had, just like them making the triangle packs to put unlimited cards in 1st ed packs. Perhaps the last of the Galaxy supply was used to print Italian cards, from the Pikachu/Pichu black star promos and some leftovers used in Italian base. The fact that they are both theme deck holos does suggest that possibly some number of theme deck sheets were printed in galaxy holo.