I mean, I don't think it's a great term/name for them either, but nothing else has caught on so that's what people say.
I also wouldn't say that Chrom was never similar to Marth. He's built off Roy who is similar to Marth, therefore also similar. Again, I'm not trying to call them direct echoes or clones but there are clear parallels and overlap in how they're designed because Roy/Lucina/Chrom were all built from Marth's move set.
The usage of these terms isn't about the fine details of development, these are fan terms about how the characters play. Like, the term shoto is used outside of Street Fighter to describe characters from other fighting games that follow a similar design. Outside of Smash, you have some people who would call a character who is derivative/inspired by of Fox or Marth a spacie or Marth. Take Rushdown Revolt for example. The character Zhurong obviously isn't a 1-to-1 copy of Marth but she's very clearly inspired by his kit/playstyle, so many refer to her as that game's Marth. Because Smash serves as the general progenitor of the platform fighter genre, the terms for character archetypes are named after its characters.
I do understand the desire for applying accurate terms when it comes to discussing the actual development side of things, but the context that I used it in was more general and half joking.