I haven't had a chance to play around with it yet, but I wonder if it **IS** intended. Consider, would the hound ever land its spell on high difficulty if it did not scale?
Obviously, scaling the entire character with difficulty would be pointless, but it may be that pets need it in order to be even marginally useful.
Other summons don't scale with difficulty though (Which is why I can't stand playing Beast Master on Tactician... Companions having 30-40% accuracy is attrocious, on top of being easily one-shot and thus gone until short rest)
That said, it's possible that they did uniquely make the Hound scale specifically to address this, especially with the fact that it costs Sorcery Points which are a Long Rest resource.
Though, it'd be strange if they intended to only make Hound scale and not every other summon in the game (Which all suffer from being kind of trash on Tactician. Low health and poor accuracy is a consistent theme across all summons, from Ranger Companions to Zombies to Elementals and Woodland Beings)
I haven't checked if the other new subclass summon (The Hexblade spectre) scales to difficulty, mostly because I seem to only ever proc the curse on the very last enemy of an encounter so don't bother summoning it (I also rarely remember to use the active curse skill). So I don't know if this is a thing for just the new subclasses, or specific to Hound only.