scuttle crab has in fine lines above it (units with 0 might can conq and hold) but i thought this was already a thing by default? so does it only work when that card is in play

No — that line is reminder text, not an ability, so it does nothing and isn't required for 0-Might units to conquer or hold.

Reminder text appears in italics and parentheses, and Rule 135.2.d.3 is explicit that "the presence, absence, or exact wording of reminder text has no effect on game function." Control of a battlefield is established simply by having Units present after a Showdown/Combat (Rule 190.4), and Might is not a factor in that check at all — a 0-Might unit conquers and holds normally (FAQ #12515).

So you were right that it was already the default. Scuttle Crab simply carries that parenthetical as a reminder because a 0-Might body looks like it "shouldn't" be able to take a battlefield — it's teaching text, and it works the same whether or not Scuttle Crab is on the board.