Not that big. A few minutes of following the instructions and you can build it in a docker container, post it as a tarball that anyone can use without needing the rest of miniconda to install.
I don't think thats true. Our compilers are complete pseudo-cross compilers (i.e. with binutils, glibc, kernel headers and their own sysroot) that can be both hosted on and always target CentOS6. This means software built with them will run on CentOS6 and will not, by default, pick up your CentOS6-incompatible headers and libraries in /usr/include and /usr/lib respectively.
None of that requires conda to be able to do. Can also build in a chroot, without needing an unconventionally configured compiler, if you want to make your binaries redistributable - which is a requirement some users will have but not all. "Easy availability of gcc 7.2 on CentOS 6" doesn't require being able to build for centos 6 from something newer, docker or chroot tools are fine for that.