Yup. Why did they do this instead of adding an ICC profile to an HEIF image - that would be at least something standards-compliant and fully capable of encoding 10-bit HLG.
Obviously with a sample we might find that an “HSP” file is just that - a renamed HEIF file.
Sadly, for compatibiity, the only way right now to get HDR/wide-gamut stills displayed on a majority of HDR/wide-gamut capable displays on the market is to encode the still to a 10-bit H.265 video with either HLG or PQ transfer function and Rec.2020 gamut. Fortunately the P frames on a still that isn’t changing encode to very small additions to the file size.