Has the NLRB ruled that Amazon is a joint employer of subcontracted delivery drivers?

No, even though recent click-bait headlines are saying this.

The NLRB itself has not made such a ruling.

An NLRB Regional Director in California has made a "merit determination" which is an early step in a process that could put the issue in front of the NLRB itself.

Amazon does not directly employ drivers who deliver packages. Amazon has contracts with companies it calls Delivery Service Partners – DSPs. These DSPs employ the drivers. There are over 3,000 DSPs employing some 275,000 drivers.

The Teamsters Union was able to unionize employees at one of the DSPs in California – Battle Tested Strategies. Teamsters wanted Amazon to negotiate a collective bargaining agreement, but Amazon refused. So Teamsters filed unfair labor practice charges.

Teamsters is arguing that Amazon is a joint employer of the Battle Tested Strategies employees. This is because Amazon exercises significant control over the drivers, such determining their routes, setting delivery targets, and monitoring their performance.

There are many steps yet to be taken – the General Counsel filing a complaint, a hearing before an administrative law judge, a hearing before the NLRB, an NLRB decision, and possible review by a US Court of Appeals.

Unless there is a change in the membership of the NLRB, I expect the Board will side with the Teamsters.

Your results may vary. If you have this issue, consult a good employment lawyer.
(Not me, I'm an arbitrator.)