הנה התבאר לך שכל מה שתקנו אחר משה נקרא דרבנן
Means to imply that all takanos made by Moshe or even that some takanos made by Moshe (to address GU’s point) are d’oraysa. A takanah is not a d’oraysa – period.
So what does the Rambam mean? In the Rambam’s world there is not a precise either/or split between d’oraysa and derabbanan the way we think of it. Categories like divrei sofrim and asmachta straddle the fence. While takanos made post-Moshe Rabeinu are all categorically dinim derabbanan, takanos made by Moshe may fall into this grey area.
The Rambam in Hil Aveilus 1:1 doesn’t say that observing seven days of aveilus is derabbaban – what he says is that it is not d’oraysa, meaning it falls into this grey area I am trying to describe. While the pasuk that describes Yosef’s aveilus cannot serve as a makor for a din d’oraysa, the very fact that there is a pasuk pushes the idea of aveilus out of the pure derabbanan category into something a little more than that.
Are there nafka minos to the categories and does the Rambam apply them consistently? I don’t know. I’m just tinkering with the ideas for now and trying to figure out what the words mean.
2) The Maharatz Chiyus (Brachos 13) asks why the Rambam never mentions the issur of calling Avraham by the name Avram. He answers by using the principle that halacha cannot be learned from events pre-mattan Torah, and therefore the issur is not binding. The Rambam’s rejection of Yosef’s mourning as a basis for dinei aveilus would be a proof to this idea.
Two old posts highlight problems I have with this approach. 1) Link #1. The Rambam in Hil Aveil does not say “ain l’meidim m’kodem mattan Torah” – he says “nitna Torah v’nitchadcha halacha.” This is a different principle (see R’ Soloveitchik’s Shiurim l’Zecher Aba Mori vol 2 p. 204). The idea of having seven days of mourning may historically have started with Yosef. However, the process of B"D formally ratifying the practice, which made it binding post-mattan Torah, changed its nature. 2) Link #2. As Rav Copperman writes in his intro to the Meshech Chochma, there is a difference between learning halacha from behavior of the Avos/Shevatim and learning halacha from the formula pesukim use to describe those same events. The former makes use of historical events as precedent; the latter makes use of the literary formulation used by the Torah. The principle of “ain l’meidim m’kodem mattan Torah” applies to the use of behavior/history as precedent, not to the use of pesukim. When the gemara makes a derasha and learns out a din that one is not permitted to call Avraham by any other name, that is based on the words of Torah -- it's not at all the same as learning aveilus from the behavior of Yosef or sheva brachos from the behavior of Lavan.