Wow!! This is just completely wrong! It seems that they (I have no idea who "they" are) have misread their references! Now I understand why you have written what you wrote. However. this is incorrect. The general page on quarks is instead correct, and I can also recommend good books if you are interested.
In a few words, the thing is that the top quark is the only one that can decay. It can decay through three possible channels, leading the production of a W boson with a bottom quark, or a down or a strange quark (the last two being rarer).
The other quarks do not decay. They form first composite states named hadrons. Then, hadrons can decay into each other, and you have many many decay modes depending on the quark content of each hadron. At the end, those decays involve the conversion of one or some of their internal quarks into other particles.
I hope this clarifies.
Cheers!