If you set it to watched during 2019, it will count towards the yearly challenge. Anything done before you join the challenge that applies will count when the challenge updates which is a periodic check.
Looking now, (
https://www.anime-planet.com/users/D3xter/challenges/2019-anime-watching-challenge) everything you have marked watch at all is shown since you joined the site this year.
If you want to clear the stuff that you didnt want this year.
> unmark everything you watched this year
> hit the reset option on the page, the challenge will clear everything shown in the challenge.
> then after that remark what you watched this year and your challenge will then accurately reflect what you have watched this year.