What is the difference between "happy to hear that" and "happy to hear this"?

Let's say I've sold a product to a customer.

A bit later the costumer tells me "wow, this is fantastic".

Now, do I better answer with "Thanks, I am glad to hear that" or "Thanks, I am glad to hear this"?

What's more correct?

I'll use that, just because I'm not the person saying it.

From my perspective, the pronoun that is introducing a clause. Let's see the situation:

(1) Costumer: Wow, this [product, which is close to me, the costumer] is fantastic.

(2) Salesman: Thanks, I am glad to hear that (the product is fantastic).

If we changed that for this in (2):

Salesman: Thanks, I am glad to hear this (the product is fantastic). -Obviously wrong.

OR

Salesman: Thanks, I am glad to hear this (statement from you). -Obviously forced.

Another possibility is that that is not introducing a clause but being used instead of a phrase.

Salesman: Thanks, I am glad to hear 'this is fantastic' [that].

Even another reason, for me, is that in (1), the speaker says this. Then, the listener can't refer to it (the product) with the same this, but with that because the speaker (Costumer) imposes his relation to the object of predication (the product), leaving the other part without the possibility to address the object in the same way (i.e., next to or in relation to the salesman).