Earlier quoted context omitted.
They're building it first and foremost for OpenBSD. If anybody's interested in using their fork, they'll create a portable version, as they do for OpenSSH, OpenSMTPD, &c. The act of stripping the library down is worthwhile though: OpenSSL has accumulated a lot of cruft over the years.
I think restructuring OpenSSL with better modularization could achieve / maintain more compatibility than just ripping things out.
All they really need to do to maintain compatibility is maintain the same API. And they're doing just that. In fact, it's in their best interests to maintain the same API as deviating in any significant way is just going to complicate porting software that uses OpenSSL over to use their implementation. So long as they do that, there's little to worry about.
And maybe some day their fork will end up becoming the dominant SSL/TLS implementation, much as OpenSSH became the dominant SSH implementation.