They won't. They are very 'serious' about their API access. You won't get the chance to try it out without a full-blown company behind you.

I've tried to setup a dev account but I was stopped as I had no company homepage and privacy statement. Oh and they call it Garmin Health.

Do you think they would change to accept data from competitor devices? I suppose they could accept the data but make the analysis/experience much worse to make Garmin devices more appealing.