Yes and no. Depending the legislation you are in or itch is in. Laws are not worldwide, also if a payment method is not reached by taxes for whatever reason. Well that.
Also, an In-store currency also works like a in-game currency. Since is not a traditional back/financial operation is not reachable by taxes, may be platform fees depending of the ToS.
So tracing the route. You use a credit card/paypal,etc to buy 'itchCoins' -> here you pay applicable taxes. You buy a game inside the store using the 'itchCoins' -> no taxes, because the operation is outside trad banking system. Want to cash out your funds? (transforming the 'itchCoins' in FIAT) -> here you pay applicable taxes.
Is something like the coins in Webtoons or the Steam wallet.