Just wanted to confirm my understanding, since I have merely observed this, but did not read it anywhere:

"that" when used as a "relative pronoun" can refer to both singular as well as plural nouns.

example, both of the folloing are correct:

The car that belongs to me needs to be repaired.
The cars that belong to me need to be repaired.

However, when used as "demonstrative pronoun", "that" can only refer to "singular" nouns.
As "demonstrative pronoun", we need to use "those"/"these" if the intent is to refer to plural nouns.

Example, following would be wrong:
Men's earnings are greater than that of women.

The correct form would be:
Men's earnings are greater than those of women.

Can someone please confirm. Also, can someone point to a link on the net that more "academically" addresses this issue.

You are quite right. There is no site I know of that might address 'this issue', because the relative pronoun has little connection with the demonstrative pronoun. By some grammarians, they are even considered different words, mere homographs, spelt the same but pronounced differently and having different grammatical functions.