why does ekko do 800 dmg with an autoattack then can dash halfway down a lane

He can'd do 800 damage with an autoattack unless he has lichbane and his passive hit on you at the same time, even then he would have to be full build and you have minimal MR to do that.

perhaps you are reading death recall wrong, it calculates all autoattacks he does to you as one

Because you built no MR and let him hit you twice already.