The Gemara search in the hava-amina a posuk for limud of the first and last
brachot mideoraita. Ve lefi col haman deomrin bracha shelefanav nilmedet
bekalvahomer miberacha aharona. Lich'oura yotze shebehava amina hakalvahomer melamed al din deoraita shel beracha rishona. Im ken, gam leachar hamaskana shesh'ar haperot o "lav bar netia" haberacha aharona shelahem yotzet lanu misvara, harey she din birkat hamazon nish'ar deoraita veim ken gam haberacha harishona tih'ye deoraita mikal vahomer. Ve kashe de'harey yadua she kol berachot rishonot hinam derabanan.
TOSFOS (DH l'Fanav Lo Kol sh'Ken) asks your question. The Hava Amina is that it was a full-fledged Kal v'Chomer. The Maskana is that it is not a real Kal v'Chomer because there is a Pircha to it, as the Gemara says earlier on 21a. (Ritva, Rashba)