They pretty much told everyone that a proper integration for OpenSSL would take years. The server API seems to be in an early review state and slowly progressing.
As the blog post mentions, the main question is not why a including a QUIC implementation in OpenSSL will take years (that’s reasonable), it’s why the only way to do QUIC using OpenSSL is to use their implementation of the whole thing. The way QUIC hooks into TLS is admittedly a little bit peculiar, but it’s in no way impossible to separate the layers. The OpenSSL devs just decided they don’t want to.
https://github.com/openssl/openssl/tree/feature/quic-server