A web host or CDN might allow arbitrary domains to be added, and arbitrary certificates to be uploaded for said domains, all without any validation. That would ... not be my favourite implementation, but if you didn't know about how the Web PKI and ACME works, that's an implementation you might come up with and not expect a whole lot of issues.
However, it's unlikely that the web host would allow you to do this for a domain already associated with an account, or a subdomain of such a domain. Unlike the first case, this would effectively allow an attacker to fully control any subdomain, so even without any Web PKI involvement, that would be a vulnerability in and of itself.
It remains to be seen what the two affected providers were actually doing, and I don't really have enough data to make a call on whether it's actually worth changing this aspect of tls-sni-02, but it's something to consider.
tls-sni-02 requires the generated self-signed certificate to contain an additional subjectAltName (SAN B) that is not send as part of the request and thus only known to the actual client, not to any host that automatically generates self-signed certificates (if such hosts exist).