Assuming the post AB 5 system essentially has rideshare workers agree to take 2-4 hour "shifts" (just a guess based on the easiest adaptation and how Amazon's delivery system works), the 45 minutes of idle time would be paid for by the company the driver agreed to work the shift for. Needless to say, such a system would heavily incentivize Uber and Lyft to avoid having drivers sitting in airport lots.

In practice they just won't drive for two companies. With minimum wage / employee status that won't hurt them at all. In practice, Uber and Lyft already don't want you keeping both apps open - they encourage you to only keep their app open with incentives.