It means absolutely nothing. Because every GPU vendor has a different notion of what a "core" is. Some count the smallest parallel execution unit as a "core" and thus boast chips with thousands of "cores", some group these smallest units into larger units and count those as a "core", which results in smaller numbers of "cores" per chip. I would guess Apple is doing the latter. I don't think that it's possible to deliver the graphics performance the M1 is delivering with just eight "cores" in terms of smallest computational units.

Yeah; Apple claims 128 "execution units" and 24,576 concurrent threads (I assume something analogous to SMT going on there, though I don't know enough about GPUs to be sure).