Smurf is a rough term to get used to with apex. It is hard to tell who truly is a smurf. In respawns eyes, a smurf is someone who purposely tanks their stats to be placed in lower-end lobbies to stomp on new players.
So if someone is level 43 as an example and in plat/diamond lobbies and kills you this is no smurf. This would be labeled as someone's alt. You are allowed to have alt accounts. If someone dies to a 43 in their ranked lobbies, you did not die to someone keeping themselves at a lower rank to stomp players. You died to someone who is of that skill level and in the proper spot until they can move on to the next ranks.
Now if someone makes a new alt account drops 20 bomb 4k then places them in higher-end lobbies they can not do that in and they go create a new account to do it again that is what is labeled a smurf. While I can understand I will most likely catch a lot of negative outcomes with this statement or thought process on it I am just writing what it was a respawn dev said when they spoke at the matter replying to a player asking what the report option was for.
Smurfs in other games seem to be looked at much differently than here in apex. A smurf over there is anyone who is good and makes a new account to smash people. Over here it's someone who does it over and over to farm low-level or new players.