DKIM and DMARC are not anti-spam techniques per se. They are used to verify that the message is authentic, and that sender is authorized to send email on behalf of the domain.

If the sender is passing as an authorized sender (DMARC aligned), then the receiver has a pretty good indication the email is legit and that the sender was delegated to sent email on behalf of the domain. If the email is then classified as spam (based on its contents), then it is easier for the receiver to choose whether to adjust the reputation of the domain (in case of DMARC alignment), or the IP (if not aligned).

A DKIM signature and DMARC alignment is no guarantee that the email passes spam filters. The whole point of DMARC is to give the receiver as much information as possible to make a confident decision on the legitimacy of the email, and the reputability of a domain.

DMARC and DKIM works both ways, if you are sending legit email (not spam), it will improve your deliverability, but if you are in fact spamming then DMARC will reduce your deliverability (as it should).