The days when there were generators instead of alternators, on small engines (single cylinder mostly) the generator doubled as the starter.or at least the beefed-up alternator - can help spin the combustion engine, including at start up.
But this is not the case with the Subaru AS/S.
That was an easy way to test a generator, just jump power to the terminals, and it would motor if all was well.
A bit of rewiring, and I guess an alternator could do the same. Not sure how they get around the alternating current vs the direct current generators produced, as far as making an alternator "motor".