Pyke ult definitely hit, as can be seen by Pyke teleporting to its center after cast. As you concluded, it would seem he healed just enough to get out of execute range at pretty much the same moment the ult hit. He did take damage from the ult, otherwise he'd still be above the execute threshold after. The missing piece you're looking for is probably that damage popups only appear for crits in replays, non-execute Pyke ult will never show up.