Yeah this is a snowball of things, like Comet being a poorly designed and overpowered card, evidence by it neither saying target nor choose, but implying it's "choose," and the designer of this cube either thinking it's funny to include Comet in the cube, or just being too inexperienced to realize it's a major power outlier for a color pairing that already exists as a power outlier.

Read the ability carefully. Comet's 4-5 ability does NOT target, it simply lets the controller CHOOSE a creature to deal damage to during the resolution. Targets are chosen when a spell or ability is put on the stack, and this is always indicated by the word "target". If there is no "target", then there is no target. Choices are made on resolution, and any responses to a spell or ability offering choices must be done without knowing what the choice will be.