All reference encoders/decoders are open source (H.264, H.265, VP8/9), so that was obvious. The distinction was never in open versus closed source anyway.
After all the development model is more or less the same, but probably with faster development and less testing/reviewing than the MPEG guys do, if libvpx traditions are any sign. So possibly there will again be few bugs in the frozen specification, like VP9/VP8 had
The only difference between these "libre" (or whatever term is currently in) codecs and H.264/H.265/... is really in the patent licensing requirements and usage royalties.