And that same line is repeated by mashpiim regarding 5 teves, bdays, etc.
From my yeshiva days I don't recall saying Tahanun on 5 teves while 18 Elul was more of a tossup.
I guess the world has changed since my Yeshivah years, and the yungeleit velt hasn't caught up...
In any case, I could at least see where someone is coming from with regards to Hey Teves, it is a חג הגאולה of sorts, and ח"י אלול being connected to the founding if Chassidus.
But what is the argument with regards to birthdays and Yimei hilula?
There is absolutely no מקור for skipping תחנון then.