SBCL eval interprets until you hit a lambda form, at which point it will compile (last I checked). But you are much more correct than who you replied to :)
Unless I'm misremembering, this is only true in REPL, which should be better read as "SBCL is actually sometimes *AoT-compiling in interactive REPL sessions!!!"; #'LOAD-ing files would AoT-compile them by default.
It didn't use to - for various reasons, evaluator inherited from CMU CL had bitrotted at one point, and for many years every form more complex than essentially single function call resulted in compilation.
Few years ago there was work to fix the evaluator and now EVAL has limited evaluation back.
This is reminding me there is the sb-fasteval feature, which I have not been testing.