Most sequels are to the first game but play a bit like it. You are right in that nothing plays exactly like RCR except downtown special which is them putting on a play.
There are sequels that play the same but with no jrpg elements, squeals that play differently and have different jrpg elements and of course the sports games.
While yes there is no full on squeal to this entry to the series other than the down town special one there are many sequals that are at least close
Now after researching this product these people aren't as ignorant as the first people who tried to do this (They did their research, some have lived in japan or are in fact in japan now) and they went ahead and worked with million corp directly. As a Die hard Kunio fan (Almost bought the underwear) I give this project my blessing, the clearly want to bridge the universes rather than make them alternate and expand on the characters in a good way. Oh and that gameplay just looks boss.
When I heard they will be porting this to japan and making it a Kunio game... SOLD! These guys are mah boys.