Salam Alaikum Ustadh, in Manhaj As Salikeen I didn't understand what that statement meant clearly, *Ijma' isnt an evidence by itself but it is an indication of an evidence that didn't reach us.

Ustadh AbdulAziz al-Haqqan

Meaning they all agree because of a hadeeth for example that didn't reach us, therefore they are all upon this saying and there isn't anyone saying something else.