The section "Creating a character label" on p. 90 of the Wayfinder Guide for Eberron said:
Dragon marks are associated with a breed, represented by a combination of variant breeds
- For humans and half-orcs, a dragon mark is a variant breed that replaces the normal
traits associated with these breeds.
- For half-elves, a dragon brand is a variant breed. You keep some of the norm
half-elf features and replaces the others by the traits associated with your mark.
- For the dwarves, elves, gnomes and halflings, the dragon mark replaces your subword.
As you can see, it is said that, for humans, the Dragon Mark is a variant version of the breed that replaces certain traits of this breed.
And under "Brand of feature making" (p.101), it is written:
The manufacturing mark is only manifested on humans. These traits replace the human trait Increasing the ability score in the Player's manual.
It lists an ASI for Dragon Mark and a number of other features.
It applies to the fundamental human race and is a new variant; it is not meant to be applied to the human variant, which is already a different variant of the human race.
Note: D & D Beyond seems to have an error on stand-alone racing lists where variant / variant dragonmark races also appear (if you have purchased the sub-races / variants themselves, not just the "content of the compendium"). Although this includes information about the dragon brand, the associated home, some examples of characters with this dragon mark and the associated traits, it seems to leave out the description of the traits (eg, your ASI) that it replaces, or if it serves as your sub-race (eg for halflings). This is true only for autonomous shopping lists; the content of the compendium itself correctly displays the complete list.
For example, for the human make mark, the human race list includes badly formatted headers: the Human Brand Mark and the House Cannith subheading (which should be a sub-section) have