First: I dislike HDMI and I don't own a TV or video device
From what I can tell HDMI v 1.3 (since 2006) supports 2560×1600 x 75Hz at 24bit, so the FUD regarding resolution is BS.
Secondly: in this case I think that HDMI Licensing, LLC might have a point. HDMI and (mini)DisplayPort are two different things. From what I understand both electrically, and at the media access layer, they are not equivalent. There are no standards defined for testing therefore *by definition* a (mini)DP to HDMI cable cannot be called an HDMI cable.
At these data rates, the connector impedance becomes an issue - the physical size differences cannot necessarily be compensated for.
AFAIK The dongle solutions (e.g. mDP to HDMI from Moshi) contains some electronics to convert the signals, hence it can be tested according to some standard.
So yeah, my professional engineering opinion is that HDMI licensing LLC might be well within their rights, and that it is probably a *good* thing that they don't allow HDMI to mDP cables to exist because *they wouldn't work properly*. A dongle (defined as having some extra electronics in it) would, and a dongle /= a cable.