In other words, had AB 5 not passed, this would likely have happened anyway as a progression towards off-loading "non-core" competencies to third parties as has continued to happen across industries (an iconic example is janitorial services which used to be handled in-house but were off-loaded starting in earnest in the early 90s?)
No, they could have done this before AB 5. They did not. This seems to say that this move would have happened if conditions allowed it and the situation was right for business.
What that means is sure AB 5 would have nixed this option. However, the trend before AB 5 was to off-load "non-core" competencies. Prop 22 re-instated the previous status, it did not introduce an altogether new status.
Prop 22 indicated the people preferred the original status to the new status introduced by AB 5, so we're back to where we started before AB 5.
Vons and Pavilions are not the same as Uber, Lyft, etc. I think they did it because they thought it was time to change their delivery logistics and were not waiting for concretization of the law around this.
Put another way, Vons' business model isn't centered around this law. It's incidental.
Companies like Albertsons who could benefit from these businesses weren't willing to commit to embracing them until there was stability in the industry. For all the faults of Prop 22, it provides stability through its incredibly high requirements for repeal or amendments. Companies like Albertsons now have the stability they desired and will embrace these rideshare companies to save a few bucks by shifting a greater financial burden onto individual drivers.
Having someone else do your deliveries and such has been a thing for a long time. Companies have gone back and forth depending on who's doing the bean counting and what they think is crucial to business. Sometimes they would split it. Some in-house some hired out.