Where to begin. Korky actually makes fill valves for Toto, but maybe the so-called "stock" Toto fill valve Stewart is referring to is the Chinese one that they also use. That's a Toto design manufactured in China. They have revised that valve a bunch, presumably to improve it as it has had issues from the start, although I imagine that it is inexpensive enough that they can afford to just replace them under warranty when they crap out. After they first introduced that "Voreto" valve, they very quickly reversed themselves and went back exclusively to Korky 528s that Korky made for them with various color caps embossed Toto. Then they rolled it out again, and used it almost-exclusively for a while; now they do some of one and some of the other.
Toto did do one thing that I am convinced they did as a result of following Terry's lead. Terry had long been recommending the 528MP as a replacement for all Toto toilets. Now, Toto no longer sells those Voreto valves as replacements. They had been stocking probably 20 different models set with the appropriate refill ratios for various of their toilets. Instead, they had Korky produce a single Universal Toto Replacement Fill Valve, model TSU99A.X, which is essentially-identical to the 528MP, in that it has the same variable refill adjuster. They did take it one step further in that it comes with a chart showing exactly how many pegs high to set it for every Toto toilet, and it also comes with a little device that will precisely set the refill adjuster according to a chart for each toilet. It may be a peg or two higher at its maximum extension (which you don't need). Otherwise, it's a 528MP.
We have the 528 in various flavors in all seven of the toilets in our home, and are very satisfied with it. It fills quietly, shuts off quietly, doesn't create water hammer. And it's easily-repairable without having to remove the valve body from the toilet. So for us it's great.
And for what it's worth, the 528T is indeed a "real Toto fill valve". It's exactly the same as the one that comes in Toto's GMax toilets (set at a 40% refill ratio, with no possibility of adjustment); it just goes into a different box when it goes to your local store. It is considered an "official OEM part", which is why it has the Toto logo on it. However, its days may be numbered because the 528MP can do everything it can do, and is useable in many more toilets, so there is no real reason to sell it separately.