@Xlev, the TL/DR answer to your question is the vendor is wrong. Not all kits decline after 2 years.
The long answer is that there is no single answer. It depends on the quality of the vendor, the quality of the kit, and the grape type.
The quality of the vendor is a criteria. I'm familiar with Winexpert (WE) and R J Spagnols (RJS), and they are pretty much top of the heap. Right now I'm drinking an 18 month old Finer Wine Kits (FWK) Chardonnay, and it's aging very nicely. There are other good quality vendors, but I don't have experience with them and cannot offer advice. OTOH, if you buy a cheap kit, well, you're buying a cheap kit.
Grade within the vendor matters. The lowest end kits from WE and RJS make good wine, but I wouldn't plan for long aging. The mid-range kits (e.g., WE Reserve) appear to be the best value. I've tried premium kits and IME the results did not match the price.
White wine kits have less shelf life, which is also true of fresh grapes. Although I've had whites last 4 years, I've also had them start decline at 15 months. The FWK Chardonnay I'm drinking now is very nice, but a WE Reserve Chardonnay was in serious decline at the same age (18 months). I'll use up the FWK in 6 months or so, as I'm not counting on a much longer shelf life. This doesn't mean it won't last longer; just that recent experience with whites gives me reason to drink it up and make more.
Reds generally have a longer shelf life than whites. Kits that include skin packs, aging oak, and/or have an ABV above 13% generally have a longer shelf life, keeping in mind vendor quality and kit quality. I agree with
@Brian55 that red kits may need 2 years for best results.