Continuing in the מאמר, the רבי connects the two preparations for מתן תורה with the two words "מדבר" and "סיני":

While "מדבר" represents the 1st preparation - the ביטול of the איד to Hashem, "סיני" represents the 2nd preparation - the perfection of working and refining oneself.

Why is perfection and completion part of the package of the ביטול preparation?

Well, what exactly is ביטול? Utter selflessness and negation of any expression? Or, perhaps it is identifying oneself - including all of one's expressions - solely as expressions of Hashem!

With this newfound appreciation of ביטול, the רבי explains why תוספות wanted to learn the גזירה שוה that tells us when the אידן began their preparations for מתן תורה from the word "מדבר" - representing total ביטול. Yet, the גמרא did not learn it from there and instead learned it from "הזה" - as תוספות explains - because indeed, in the word "הזה" - representing expression - is where true ביטול is to be found!