-snip-
The mixer that is going to advertise their project on this forum is a Bitcoin mixer and that's the main reason why they're paying with Bitcoin. This forum allows Bitcoin payments and that's why it's not fair to come up with such questions. There can be many bounty programs that are meant to promote altcoins and they pay with altcoins if you want one of those then you'll get one on this forum by looking at bounty board but this specific campaign is for promotion of a Bitcoin mixer and that's why the pay will always be with Bitcoin.
What about your opinions in the following campaign threads for other Bitcoin mixer services:
I'm not sure about other members but I'm in favor of altcoin payments in such times because currently the Bitcoin network is highly congested and the congestion may last for sometime and meanwhile we may happily receive the payments via an altcoin like Lite Coin, XMR, or maybe via a stable coin like USDT. I prefer XMR as that's the best choice but yeah LTC is quite good too.
Under certain conditions, such as Bitcoin transaction fees, which are relatively high, I think the option to pay using one of the altcoins could be implemented.
Even though
Mixero is a Bitcoin Mixing service, it utilizes the privacy features of XMR mixing.

In my opinion, even if the payment campaign uses altcoins such as XMR/Monero (for the time being), it can be done.
I don't see any special rules requiring payment with Bitcoin on this board.
-cmiiw-