The question is based on a false premise!
There is no connection between Tetzave and Moshe Rabbeinu's Yahrzeit as far as setting the calendar is concerned.
While it often happens that 7-Adar is during the week of the Tetzave Torah reading, it sometimes happens not that way.
E.g. in 1977 2001 7-Adar was on Friday before Terumah. In 2029 and 2032 it will be 3 days after Tetzave.
But, even ignoring facts on the ground, the Gemara in Megila 29b discussed Tetzave being Shabbat Shekalim - i.e. before (or on) Rosh Chodesh Adar - so that 7-Adar would be on the Thursday after Tetzave.
איתמר חל להיות בואתה תצוה אמר רבי יצחק נפחא קרו שיתא מואתה תצוה עד כי תשא וחד מכי תשא עד ועשית
An Amoraic dispute was stated: If the Shabbat on which the portion of Shekalim is to be read occurs on the Shabbat in which the regular weekly portion is “Tetzave” (Exodus 27:20–30:10), what should be done?
Rabbi Yitzḥak Nappaḥa said: Six people read from the portion “Tetzave” until but not including the weekly portion of “Ki Sisa” (Exodus 27:20–30:10), and one person reads the portion of Shekalim from “When you take the count,” until but not including the verse: “And you shall make a copper laver” (Exodus 30:11–16).
Then the Gemara discusses the case of Shekalim being on Ki Sisa - so 7-Adar and Tetzave happened 2 weeks before Rosh Chodesh Adar - presumably in Adar-1. This does not bother the Gemara.
חל להיות בכי תשא עצמה א"ר יצחק נפחא קרו שיתא מן ועשית עד ויקהל וחד קרי מכי תשא עד ועשית
The Gemara considers a similar case: If the Shabbat on which Shekalim is to be read occurs on the Shabbat on which the regular portion itself begins with “When you take the count,” what should be done?
Conclusion: It's a cute Dvar Torah about Tetzave coinciding with 7-Adar and Moshe's Yahrzeit, but it's a coincidence that happens a lot, but not every year.