Yes, I should think that the sentient trees in the Old Forest were indeed Huorns of some sort. We know that originally Ents lived in forests apart from just Fangorn (Tasarinan, the woods of the Ered Luin, the forests of Cuivenen etc), so really a Huorn (originally an Ent) could be found anywhere; but since the Old Forest was once part of Fangorn anyway, the likelihood of the remnants of Ents being there is high. Probably their small population and the hemmed in size of their realm forced them to give up being true Ents, and revert to their tree-ish forms (probably ultimately the fate of all the Onodrim, in the face of the Dominion of Men). Perhaps the occasional one woke up from time to time, as did a particular old Ent that Treebeard knew, and went wondering; the 'giant' that Hal Gamgee saw.
The existence of Huorns, i.e. fallen-asleep-Ents, in the Old Forest raises an interesting possibility. Could they in fact have been the long-lost Entwives? Treebeard remarks that the Shire is 'country they would like' -- settling a small forest on the edge of the Shire (tamed but still natural and living countryside, full of nice gardens etc which is what the Entwives went in for) may have been just to their liking, only they couldn't sustain it and all fell asleep.
The fact that it was Old Man Willow complexifies this debate, of course.
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