Well, that's not a filter, you're using the field "Updated At" as a flow step condition. You set the condition as: if the person is "Updated At in past Y weeks" Do Nothing, and in the Default Choice, update their Behavior Score by subtracting as many points as you like.

Also, instead of using the "Updated At" field for scoring, I'd rather advise using meaningful person interactions to score (positively/negatively) people. E.g., for score decay, you could consider: if a person hasn't submitted a form in the past X weeks, hasn't visited your website in the past X weeks, or has Unsubscribed/Opted-Out, etc.
The "Updated At" field would get updated for any updates on the person record, and these updates need not be person interactions. They could very well be operational updates (e.g., add to a list, system field updates, etc.). Even Opt-out, Unsubscribed are also counted as updates. So the campaign you're trying to build, a person who recently Unsubscribed, or a person who was added to an operational list (not due to any of their interactions/actions) won't flow through the flow, and hence, their score won't be decayed, even though it makes sense to decay their score.