Walker is part of what is currently the best defence in the Prem, will most likely be part of the England squad for Euro 2016 and is one of the Prem's better RBs.
As for Pogba, the discussion is not about what he's done since leaving United (he is, after all, 3 years older than Alli). It's about comparing Pogba when he was the same age as Alli is now ... and in that comparison Alli comes out favourably.
Alli's a starting regular in a strong Prem team, he's scored 4 league goals and made several assists, and he's part of a CM combo that has dominated the CMs of City, Arsenal, Chelsea and others. Pogba had reached none of these levels at the age of 19.