Usage of whom?

"Whom" should be used instead of "who" after a preposition.
e.g. With whom did you go?
I have a number of American relatives, most of whom live in Texas.

Kasia@MasterGMAT wrote:"Whom" should be used instead of "who" after a preposition.
e.g. With whom did you go?
I have a number of American relatives, most of whom live in Texas.

But how about if we follow the link:

https://www.beatthegmat.com/mba/2009/10/ ... correction

Shouldn't this make the usage of who correct in the above sentence?

Hi Ankit , I went through the post you mentioned.
Based on what I could gather:

Laos has a land area comparable to that of Great Britain but a population of only 4 million people, many of whom are members of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.

here, "whom" can be replaced by "them" and not by "they". So , whom is correct.
Try replacing "whom" by "they", the sentence becomes awkard.

Hope this helps!