Exactly. Last I checked, the Kremlings were the only villains shown that have been on hiatus for so long: no cameos, no remakes, nothing. They could have used any other villains from DKC Returns in the Kremlings' place. There are non-Tiki enemies, too, if you think "enemy variety" is a strong argument. But no, they revived an enemy race that was last seen 6 years ago. That is significant.
The Kid Icarus enemies and the green shell from Smash Bros N64 are not analogous counter-points because unlike the Kremlings, they arerelevant/recent. The counterparts to the Uprising enemies and the Koopa shell for the DK franchise in SSB4 are the Tikis. Because of this "recency" bias (just look at the Octorok designs -- they are pulled from the last major Zelda console game), their appearance and the appearance of all other enemy characters is expected/inconsequential. To reiterate, the Kremlings' disappearance from their home series makes their revival in Smash Bros. distinct from all other enemies we've seen.
And yes, like the OP and others have pointed out, the Kremlings do not guarantee K. Rool. They do, however, make him more plausible given that his generic race is being represented despite their complete removal from the recent DKC installments. In fact, even those that think we should minimise the importance of the Kremlings appearance concede this point. Surely, if the generic enemy race is "relevant" enough to appear in Smash Bros. (even if just for "enemy diversity"), their leader (an actual specific character with a notable history and fan following) is surely "relevant" enough to be considered for the game himself.
