You are right. Many people will probably see the advertisement and will put their money on 1xbit. They will consider 1xbit as a trusted gambling website only because they (wrongly) believe that a service promoted by a reputable wallet like mycelium is trustworthy.
It's clear why Mycelium is promoting 1xbit. They only think about money.

If a custodial wallet or an exchange promotes a scam, I would immediately stop using it. Since mycelium is open-source and noncustodial, at least I know my fund is secure. This is what I was saying in my previous post.
Of course, you should no longer use mycelium if security of your fund isn't your only concern. By using mycelium, you help them have more users and earn more money from such advertisements.