Nothing ... except compliance.

The idea comes from finance -- to require collusion to execute a fraud. It's not perfect, but it's something.

Maybe I should rephrase that: Is it impossible for a company that defines Dev+Ops as a single responsibility to be compliant?

Not impossible. even in a prescriptive framework like ISO 27001, adequate SOD is a judgement call between you and the auditor. Generally speaking, if a single dev can push a code change to prod, in a way that would escape audit or not require a second pair of eyes, that would not be compliant. So if a dev writing code, also manages the deploy environment, that may not pass muster.

But it's not that cut and dried. There are degrees of rigor.

No. Assuming a well configured continuous deployment type environment; you just need to have peer review on code before it can hit production, and you need to have controls in place over the who, what and when of elevated access to production being granted

This all breaks down as soon as audit realise the Devops team is also admin of the ci/cd stack and therefore all controls put in place to make it harder for a single actor to do bad stuff can be bypassed via this all powerful system.

It seems like the description is vague enough that this is entirely up to whoever gives you your certification.