Since you haven’t responded, I will assume you are referring to this:
http://hebrewbooks.org/pdfpager.aspx?req=15142&st=&pgnum=262&hilite=
I will make the following observation – he is referring to this case:
אם הוא רואה שאחד רוצה להכנס לחנות לקנות סחורה אצל אחד
In this case specifically, he is causing a (somewhat) direct loss to the owner. This is not the case with a general discussion (online or otherwise).
Also, you wrote, Overcharging is defined halachically as charging a certain amount more than the norm. .
That’s not the case he is referring to, though, when he says it’s אסור. In a case where there is a violation of אונאה, it would be permitted (actually, a mitzvah) to tell, even where the buyer is ready to walk into the store.
He is referring to a case where there is not a (definite) violation of אונאה.