It doesn't count as cannon.
Both Manga/Anime follow the same scripts and outliners, with only slight alterations on both sides, Manga would not skip cannon arcs such as when Goten/Trunks followed monaka and ssj3 goten negged by base vegeta or Beerus dressed as monaka vs Goku and such if they were actually part of the cannon story line, Even then the manga doesn't even reference these at all nor hint they actually happened. Actual cannon content/story arcs is shown in Both Manga/Anime, if one is only shown in the anime and has zero mention or references in the manga then it's not cannon.
Regardless of the anime coming out first it still needs to fill time as it's Ep's come out weekly and so it doesn't end up getting a full arc or two ahead of the manga, while the manga can strictly keep to the script because it's a once a month thing and has no need for filler.
As for the maron thing, I'm pretty sure that scene between 17&18 was in reference to her, Hence 18's strong reaction to it.
I would agree if not for the fact that the Resurrection of F arc wasn't completely skipped in the manga and outright contradicted by the manga's later notions. While I do admit that the "filler" arcs in super may seem that way, there seems to be a complete disconnect between the manga and the anime in terms of their story events, for example, Goku fighting off Merged Zamasu in mastered super saiyan blue until his hour ran out, or the mere fact that Merged Zamasu had a time limit at all, in comparison to the anime where Trunks deals the final blow and Zamasu merges with the universe and all that. If we followed the manga exclusively then the anime's entire trunks arc end would be considered filler since it doesn't match up with the manga.
In the case of Super, both stories follow their own personal canon based off of Toriyama's outline and it's up to you to choose which one is canon and since the anime is ahead and it has been stated many times that the manga is used to promote the anime then guess which one most people would consider canon.
To the Maron thing, that seemed more like a tongue in cheek reference to her than an outright mention since there's no proof based off that scene that she exists outside of a typically android 18 response.
Not that this has to do anything with Boruto, just giving an example of how this wouldn't be the case.