GAON: “However, he does agree that according to the MB’s understanding of the Rambam (as per Magid Mishnah etc) we would at least be “m’tzarif” the shitas ‘Rabanon’ of ‘Lo Asi Rabim um’Vatlin Mechtzos” to the shitos of Shishim Ribo. The only issue he has is with the GR”A, and on the same note, the Demasek Eliezer on the Biur haGr”a (364) is definitely NOT like R”A on the issue of mefulash.”
If you continue on in that same seif katan (10) you would see that Rav Aharon disagrees with the MB, and argues that even the Rambam would require delasos.
I agree the Demasek Eliezer is in opposition to Rav Aharon’s understanding of the Gra. Furthermore, the Gra always sources from a Gemara, so I really don’t understand Rav Ahraon’s proof regarding the mare makom that the Gra cites.