Just because your IPv6 connection is bad, doesn't mean everyone has a bad IPv6 connection.
How you describe how your DNS works is also specific to an implementation, not how DNS works in general.
But yes, LFS does not support IPv6, so if your router only gives out IPv6 it's not going to work. If it gives out both IPv4 and IPv6, the IPv6 part will have absolutely no consequence for LFS.
On the other hand, if your ISP is reporting that "maybe LFS is not prepared for IPv6", yes, that's true, however they may be using CGNAT for IPv4, which could well be causing issues. But that's not IPv6's fault, not LFS.