Dr. Mario should also probably be counted as an Echo Fighter, but the website gives him normal billing.
It's because Dr. Mario has a different down-B and his moves have slightly different properties in regards to speed and damage, whereas Lucina's differences are smaller. I got the impression that Simon and Richter are identical.
But with Chrom having Ike's up-b, I see no reason to not include Doc as an echo imo
I'd assume they're basing it on the fact that even if Chrom's up-b is different than Roy's, it's still echoing another fighter, whereas Doc's down-B isn't
...Luigi? ---
NNID, PSN, and GT: Waluigi1 DD:DA Pawn: Xekoben the support mage