1. The Pnei Yehoshua here understands that this is indeed found in the Tosefta. However, the Turei Even (end of DH Aval Im Machru) writes that it is not in fact a Tosefta but instead is a "Tosefes," an addition from a scholar, and in reality this addition is a mistake.

2. The reason why the Turei Even says that it is a mistake is that this "Tosefta" is contradicted by the Gemara in Menachos (99a) which learns the opposite way from the "Tosefta." The Gemara in Menachos there states that the concept of "Ein Moridin" -- we may not descend in Kedushah -- is learned from the verse that describes Moshe Rabeinu's construction of the Mishkan, while the concept of "Ma'alin ba'Kadosh" is derived from the pans of Korach. This is the opposite of what the "Tosefta" states. While the Pnei Yehoshua attempts to reconcile the two sources, the Turei Even understands that it is clear that an error was made.

3. I found that Teshuvos Afarkesta d'Anya (of which the first volume was published in 1940 by the author Rav Dovid Shparber), part 2, Orach Chaim ch. 33 (DH K'Z Hineh Kama) writes that because the Pnei Yehoshua thought that this is a Tosefta, he made special efforts to attempt to reconcile it with the Gemara in Menachos. However, the Pnei Yehoshua writes that he looked for this Tosefta but did not find it. He agrees with the Turei Even that it is a mistaken addition in the text of Rashi and supports this argument by pointing out that the statement in Rashi concludes "Ad Kan," which indicates that this is the end of some later addition made in the text of Rashi. Teshuvos Afarkesta d'Anya adds that it is not uncommon to find such additions in Rashi and they conclude, as in our case, with the words "Ad Kan."

Kol Tuv,

Dovid Bloom