The the existence of a DMARC assertion record in the DNS changes how the MTA (the receiving system) assesses the authentication of the message and authorization of the sender with SPF and/or DKIM, as per rfc7489 (and now also rfc9989).
No DMARC results in the MTA falling back to the original assessment method as defined in the original SPF and DKIM RFCs. These RFCs have design flaws and don't define the assessment flow all that well, they leave a lot up to the MTA's digression/interpretation.
So, the existence of a DMARC record with p=none is fundamentally different over not publishing a DMARC assertion record at all. It's just that with p=none you don't enforce the MTA to reject the email if it fails authentication/authorization, you're leaving it up to the MTAs digression.