Can I use an 8" dado blade set in a 10" table saw?
There is NO problem with using an 8 inch dado blade on a 10 inch saw. In fact, it is quite common to do so. I use an 8 inch stack on my Delta contractor's saw with no problems, and before that, I used an 8 inch wobble dado.

You certainly would not want to use a 12 inch blade on a 10 inch saw, as that large of a blade might have clearance problems as it is adjusted. Use of a larger blade also causes somewhat larger tip speeds, so that might potentially cause problems. (Unlikely, but possible, since the differential in tip speed would be only 20%.) Perhaps these issues are what the author meant when they made that statement.

In going to a smaller blade diameter though, the only question is whether that blade will be able to reach the surface of your table. On an 8 inch blade there is no problem.