Doesn't matter who they are employed by. There won't be any big push by anyone to change the status quo. Only a big union seeking to spend many years and many millions could change anything. A few drivers of an ISP might try to organize, or maybe some of every ISP in a terminal and could be successful against some or all of the ISPs, but the ISP doesn't have any resources to pay union benefits of wages. So the ISP would either 1:hire scabs or 2:fail to service his routes, and lose his contract. Those drivers would be out of a job, and the current NLRB wouldn't step in to say fedex was a co-employer.
The NLRB is going to deny a unions claim that the drivers are fedex employees, so the unions would need to organize each ISPs drivers separately. A union might even get more than half a terminals drivers voting to unionize but depending on how the vote is spread out, maybe only a small % ISPs actually face a loss to the union. Kind of like how Hillary won the general election but lost the presidency because of where the votes were.
In the unlikely event that fedex stepped in directly and fired a ISP who had drivers trying to unionize, that ISP can't make much of a legal claim for damages. The ISP is going to lose his ability to service the contract sooner or later anyway when facing a unionization effort, so fedex has no reason to step in. Other contractor/ISPs won't step in to join any legal fight- they have too much invested at risk. And the ISP is partly designed to make it unattractive to the unions to try to organize drivers if they have to organize one ISP at a time. It could MAYBE be done, but very expensive and very long term.
The unions would be more likely to target an Amazon ADP if they do end up with 30-60 routes requiring maybe 100 employees.
There is no real point in talking about whether drivers are employees anymore. They ARE employees. Moot point, as I said, and who employs them is moot too. They have the same rights and legal protections that every employee has already. The NLRB won't be involved unless a unionization effort is somehow thwarted by fedex.
Fedex will likely just go ahead and payoff any liability a driver might cause if the ISP is unable to pay to keep it out of the courts. For example- an employee of an ISP with a criminal background, or bad driving record slips through the system and kills a busload of kids in an accident, or forces his way into a customers home on a delivery and rapes a woman, fedex could and should be held liable. They will probably pay it off rather than fight to prove they shouldn't be liable. It's already been found in court that fedex has used this model to avoid liability and confuse the issue.
Fedex saves so much now with this model, that an occasional payoff to protect it means little. Attorneys don't want to fight a class action just to prove some claim of employment status for ISPs. They learned their lesson with the last class actions that went on for for at least 12 years.
