is it only possible to download the games on the racial justice bundle from the websuite and not from the itch desktop app?

i added some games to a collection but when i go to them in the app it says 'buy now' instead of download

You can download them from the app, but it will show the "buy" button on the page unless you've already added it to your library. 

You can access the list of games to download from by clicking Explore > click the carat next to your profile in the top right > My Library > My Purchases on the left > Bundles > Bundle for Racial Justice and Equality. 

Clicking the download button on any of the games will add it as "owned" in your library regardless of wether or not you actually download the game at that moment or not.