Yes of course the work can be done in a day's time. I've done it myself for a HOTS april fools and made my own Hogger model in a day out of my already existing custom Gnoll model.
https://www.reddit.com/r/heroesofthe..._fools_reveal/
But working in fan content and working at a game studio where work has to go through formal systems of approval is a very different thing. There's no one formally checking over fan work and getting things to be iterated and changed; it's in full control of the artist who can interpret it at will and choose whenever to consider the model complete. That's what you're missing here.
In any professional modelling setting for a big company like Blizzard, hardly anything will just take a day, because everything goes through iterations and improvements, and we're not talking about people using the first model they finish after a solid day's worth of work even if it's really good. It still has to be approved before things even get moved forward into rigging and texturing.
That is why the assumption that the Hogger model is merely a reaction to fan demand is quite unrealistic. The model has to have been planned months in advance in order for us to actually see the final product appear now. It actually can not be applied as some knee jerk reaction that they whipped up between patches because they saw some internet dweebs talking about it. It doesn't actually make sense if you actually know what a model goes through in order to make it to the pages of WoWhead and MMO champion. There are countless hours and days and weks of work you don't see going back and forth on the details which you never knew had to be fixed or changed.
And just taking a look at this Hogger model right now, I can see the armor/clothing is modelled, the straps are not merely textures but would have had to be high-poly modelled to fit with the Rings and they would have generated textures off that high poly sculpt, the top of the mane looks longer and mangier, the scruffs on his jowls are a different pattern than on the regular gnoll, the rips on his pants are different than the pants of the gnoll, and the overall pattern of fur is different which means they may have also gone through a whole new fur treatment in the high poly sculpt. The key takeaway is that the more visual changes there are, the more likely there would have been back-and-forth iteration/approval process done before this bad boy moves on to the next steps.
With this many visual differences, the approval process would not have been as simple as a day of work. As I said, this is far from simply a mere recolor, and taking longer-than-expected is literally why there was a placeholder for him in the first place.