Well I don't have enough experience with mold to to know one way or another, but it just brushes off and wipes away with a little alcohol far easier than any any kind of mold or mildew I've ever dealt with before. If it is mold then that's a plus because it is extremely easy to deal with.
Warehouse humidity is a hypothesis but I think it's notable than nothing else inside or outside of the box was affected. There's no way mold came in from the outside world -- the spores would have had to be on the bag material when it was packaged up and it's more like prolonged lack of exposure allowed them to not die rather than an ideal environment allowed them to thrive.
When you talk about moldy product from humid storage, I expect to find surface rust on metal parts, discolored and deformed cardboard packaging, crumpled and water-stained paper, etc. At least in the case of my meter, you would never get the impression from the packaging that it spent any time at all in humid storage. After reading this thread, I was expecting far worse...
I find it difficult to believe that a third party seller would rummage through several of the probe kits to go after the 19mm tips, yet leave the leads and all the 19mm accessories behind. It's far more likely that it left the factory that way, which points to a serious process deficiency. It's not about whether or not they have a legal obligation to anybody -- it's about PR and brand positioning to maintain their assertion that they are #1 in the T&M industry.
I think we were all suitably impressed when Fluke
made good with a bunch of people who weren't even their customers. They were willing to spend $30k to prevent their name from being associated with a bad thought because somebody over there understands marketing and brand building.
Legally, Keysight can do whatever they want but this forum attracts a lot of eyes and their handheld DMM reputation is not doing so well after the assembly and inspection disaster, moldmaster 9000 carry cases, and Russian roulette probe kits. There are two ways to see this type of situation from a branding and management point of view: Either you see this as a need to shift the blame around until you aren't responsible, which does nothing to change people's perception about the company, or you see it as an opportunity to stand behind your product and make good on your marketing rhetoric.