Yes, unfortunately Meta credits are for use in Horizon Worlds to buy items. I'm told there are only a very few game developers in the Store that accepts Meta credits and I have never seen an app on the store accepting them.
Not wishing to give you more bad news but from the end of March Meta credits are discontinued, so I'm surprised you could still buy them. When I went to the site I confirmed that Meta Credits disappeared totally from Horizon + as a, form of payment. Here is the statement from Meta.
"Meta Horizon Plus (MH+)
Perks: By March 31, 2026, Horizon-specific perks — including Meta Credits, Digital Clothing, Avatars and In-World Purchases — will be removed from the subscription.
They might be still available in the surviving Worlds but you can't buy games. You could try contacting Meta Support through the Horizon app, explain your problem and they might be able to help.