DY:
Great question, and great reference.
One can sip wine all day and fulfill what this sefer suggests. I offer a few points to help understand why it is included in SA under the halachos of Seuda.
According to the Megillah, there are FOUR mitzvos on Purim, not 5 or more. They are reading Megillah, Matanos Lo’evyonim, Mishloach Manos, and Seuda. The Megillah, written by Mordechai and Esther, al pi Ruach Hakodesh, did not specify others. There are many things that became part of the celebration mood of Simcha, including the dressing up in costumes, etc. And the Chachomim proposed חייב אינש לאבסומי. These are not additional mitzvos, but are characterizations of the existing ones. This is not its own mitzvah, but an aspect of seuda. Whether this means even not during the meal, as suggested in this sefer, or not, is irrelevant. It remains a part of the seuda, and is not its own mitzvah. And I can repeat my position that if someone is looking to be “machmir” on mitzvos of Purim, begin with the ones spelled out in the Megillah.
Next, the drinking that is done on Leil Purim is not part of the mitzvah of Seuda. It can be Simcha if it remains within limits. The drunken revelry that often fills the streets is not a mitzvah at all, and is against Torah value. I am not against imbibing to celebrate in a manner that is a source of pride to HKB”H. Yelling in the streets ain’t it. Making a night of drinking risks being unable to respectfully fulfill the mitzvos of Purim by day. And the sefer you referenced here speaks openly about how drinking never exempts anyone from mitzvos, except after the fact, and bringing oneself to that state is an aveiroh.