Ok, I had a talk with @kvas-damian and did some additional research:
1) WikiaHubs.wikiamobile.scss is no longer used, so it's removed along with Hooks - only one wikia is using wgEnableWikiaHubsExt atm and it's http://corporate.mpb-test.wikia.com/ - a test wiki.
2) WikiaHubsV2Mobile.scss is linked in /WikiaHubsV2/WikiaHubsV2Controller.class.php:60: $this->response->addAsset('wikiahubs_v2_scss_mobile')