So Merlin is like the representation of everything that makes magic awesome for Snape and omg Merlin. It's kind of like meeting a childhood hero in person and Merlin hasn't disappointed yet. He's pretty much exactly how Snape imagined him to be only... young. (Snape is giving him slack on the magical power because, well, he is still learning XD)

On the other hand Snape is cautious about revealing too much and possibly changing history. On the other other hand, Snape would love to teach Merlin a little. Because how many people can claim to have taught Merlin? (and he would try to be more forgiving (and probably fail a lot) because magic has progressed since Merlin's time)

So yes, a lot of omg Merlin, it's Merlin! on Snape's part.