Not true, at least in my experiences.
I have a bunch of email accounts which have SPF and DKIM, but are missing the DMARC line - I've had no deliverability problems at all, until this weekend, and now only with Outlook. Emails to Gmail accounts go through just fine.
The combination of: √ Valid SPF √ Valid DKIM signature X Missing DMARC policy
Is clearly ambiguous. Since a DMARC of "p=none" (in the context of valid SPF and DKIM) really adds no information, so interpreting "missing DMARC" as "p=none" seems reasonable. Sounds like Microsoft was taking that approach until this weekend.
I wonder if there's an actual written standard?
Most receivers have chosen to treat this as p=none for the last decade (Microsoft included). This is changing to now so that mail with no record is simply refused. Nothing in the standard prohibits this.
a DMARC of p=none might be seen as ambiguous - SPF and DKIM have their own jobs and they work. However, the point of DMARC is not a new way of enforcing mail authentication. Instead it is a way of making certain that the mail is internally consistent and furthermore provides two additional things.
1. it indicates if the domain owner has thoroughly gotten their mail to authenticate (in which case they adopt a p=reject), and
2. it provides a feedback mechanism that allows the domain owner to know if they are seeing authentication and deliverability problems in such a way that they can fix those broken mail flows.