Do you need to deseed the grapes?

Disclaimer: I have not made this recipe nor used the two grape varietals it calls for.

I do live in California and have used many grapes in many dishes both table grapes as well as winemaking grapes. In my own experience, I don't think that a quick blend of whole grapes quickly strained will generate a lot of bitterness.

It's worth pointing out that the initial fermentation typically lasts about a week or so. In the case of many red wines, the skins, seeds, and often stems are crushed and in full contact with the must. Traditionally for many white wines, the clusters are left whole without crushing.

Based on the handful of commenters who actually executed this recipe (presumably per the instructions), I'd be fairly confident in proceeding with the recipe instructions as written.

Ultimately I will come down to your taste buds and those who will be joining you at your dinner table.

This is a quick sorbet and you should be able to judge the bitterness by tasting the puree before you start the freezing process. It's not like wine which takes months and often undergoes two separate fermentations (primary and malolactic).

This is frozen grape juice from mashed fruit.