I think that is the case.
HEIF is a image format using HEVC image coding for the best compression ratios.
libheif uses libde265 for the actual image decoding and x265 for encoding.
Alternative codecs for, e.g., AVC and JPEG can be provided as plugins.
libde265 is an open source implementation of the H.265 video codec.
It is written from scratch for simplicity and efficiency. Its simple
API makes it easy to integrate it into other software.
H.265 is pattent cant be on Fedora
What could be done is for someone interested it to submit a gthumb-freeworld alternate build with HEIF support enabled and maintain it in rpmfusion.
we also could think in a imagemagick-freeworld-7.x.x