Likely Rubles. "The issuing and circulation in Ukraine of currencies other than the hryvnia are expressly prohibited by Ukrainian law."
https://bank.gov.ua/en/news/all/zaprovadjennya-obigu-rosiysk...
https://bank.gov.ua/en/news/all/zaprovadjennya-obigu-rosiysk...
So this statement is extremely confusing because it completely contradicts reality and official bank operation there.
BTW, after the 2014 war started, I visited both Kiev and Moscow quite a bit. RUB folded 3x almost overnight after Obama's sanctions. Most well-off people and the businesses they visit, simply switched to dollars. This did not happen in Ukraine because no one had dollars. What happened instead is the many Ukranians who took out bank loans in dollars, had their lives destroyed.