"This/That is, " used at the beginning of a sentence to clarify a concept from the previous sentence

According to an English native speaker who works with me, the "This is" bit in the following sentence should be replaced by "That is":

In fact, the feature space need not be unique. This is, for a given function, there might be various valid feature spaces.

However, I have the feeling I have seen this usage of "This is" in the scientific literature.

What do you think? Should I use "This" or "That" here?

"That is" is an idiomatic phrase that has a fixed meaning. That is to say, it is used to clarify what was said. The "that" does not directly refer to the previous statement. Because it is an idiom, you can't use "this".

that is, (by way of explanation, clarification, or an example); more accurately: I read the book, that is, I read most of it. I believe his account of the story, that is to say, I have no reason to doubt it.

From Dictionary.com