Is it true that Radiant Pokémon are counted as "rule box Pokémon"?
Radiant Pokémon are rule box pokemon, because they have a rule box that reads "Radiant Pokémon Rule". For example, Radiant Greninja:
You can see the "Radiant Pokémon Rule" just below the picture of the Pokémon.
This is further clarified in the Pokémon TCG Rulebook in appendix 5:
All Radiant Pokémon appear with their special Shiny coloration and have the Radiant Pokémon Rule, which says you can’t have more than 1 Radiant Pokémon in your deck. A Radiant Pokémon is considered a Pokémon with a Rule Box. Note that “Radiant” is part of the Pokémon’s name, so your deck could include 4 Greninja and 1 Radiant Greninja. Radiant Pokémon are always Basic Pokémon and do not evolve from or into any other card.
Yes, some of the Radiant Pokémon are Rule Box Pokémon.
Rule Box refers to any Pokémon that has a text box that says the word “rule,” such as “V rule” or “TAG TEAM rule.” If your radiant Pokémon has that, it is rule Box one.
Prism Star Pokémon
TAG TEAM Pokémon
Mega Evolution Pokémon