I assume you are referring to the Asmachta mentioned in Tosfos DH Nichnesu. From what Tosfos writes later, in DH Haza'ah, it seems that he means to say the Perishah of the Kohen Gadol is only mid'Rabanan. Although a verse is quoted as its source, that is only an "Asmachta," a verse that the Rabanan used to "support" or "find a hint of" what they instituted.

I am including a copy of what we wrote in the Background to the Daf on Maseches Yevamos 52b, about "Asmachta". I hope this is helpful,

-Mordecai

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17) [line 24] ASMACHTA

(a) At times, when Chazal make a Derashah (extrapolate a Halachah or other teaching) from a word in the Torah, it happens that the Halachah or teaching is not mid'Oraisa at all, but rather mid'Rabanan. When this happens, the Gemara usually states that the Halachah is mid'Rabanan, and "Kera Asmachta b'Alma," i.e. that the verse is only cited as a "support" for the Halachah mid'Rabanan, but its source is not actually from the Torah. (TOSFOS to Bava Basra 66b DH Michlal d'She'ivah writes that in many instances, Derashos of Chazal in the Midreshei Halachah, such as Toras Kohanim, which appear to be from the Torah, are only Asmachta'os.)

(b) A second type of Asmachta applies even to a Halachah which actually is mid'Oraisa. When Chazal find a hint in the Torah to a Halachah which has its basis in the Oral Tradition, they call this an Asmachta as well (Eruvin 5a, Chulin 77a).

(c) The Rishonim argue as to the reason why Chazal, in these instances, used verses to support their teachings.

1. From the words of the RAMBAM (Introduction to his Perush ha'Mishnayos) it appears that Asmachta'os are only mnemonic devices. (It is possible that he writes this only with regard to the latter type of Asmachta, Asmachta'os for Isurei Torah.)

2. MAHARIL (in Likutei Maharil) writes that Chazal used the device of Asmachta in order to make people regard certain Halachos mid'Rabanan as if they were actually mid'Oraisa, so that they should not treat them lightly.

3. The RITVA (to Rosh Hashanah 16a, see Be'er ha'Golah of the MAHARAL, Be'er 1) states that when Chazal present an Asmachta, it means that the Torah meant to suggest that it is fitting to implement such a Halachah, but that it did not choose to make it obligatory. The Torah empowered the Chachamim to enact it should they see the need for it arise. Similarly, the SHELAH (in Torah she'Be'al Peh, entry titled "Rabanan") writes that when the Chachamim utilized a hint from a verse, it means that they learned a particular approach of reasoning from this verse. Accordingly, it appeared to them that there was a need to decree this particular Halachah.

4. The MESHECH CHOCHMAH (Parshas Shoftim) claims that when Chazal present an Asmachta, it means that after Chazal instituted a particular Halachah or enacted a particular decree, they studied the Torah and found that the Torah had already hinted to that future decree in its eternal wisdom.

(d) There are those who write that a Halachah mid'Rabanan that is learned from an Asmachta, and which has a hint in the apparent meaning of the verses, is more stringent than an Isur mid'Rabanan for which an Asmachta from the Torah is not offered. These Halachos were given the status of Halachos of the Torah in certain respects, for example. with regard to the requirement to be stringent in the case of a Safek (PRI MEGADIM, Introduction to Shulchan Aruch Orach Chayim 1:2:d)