How technical should one be if they are interested in being a Developer Product Manager?

As a Product Manager, you need deep knowledge of the personas you serve. To build great products for developers, you don't need to be a developer, but you need to understand their challenges, motivations, and day-to-day work. I encourage product managers to use the products they build so that they truly understand their users' experience. This means that product managers for developer products need to be more familiar with software than product managers for other types of products.

Quite technical, because you cannot have the requisite level of empathy for the target user, not to mention be credible with your engineering team if you aren't. Now, I do think that a mistake that PM hiring managers for devtools often make is trying to optimize for specific knowledge in a particular domain (e.g. if you're an observability company, trying to only find PMs that already know monitoring/observability). This is unnecessary. What I look for are transferable technical skills: have they been a software engineer/devops/SRE before? Or worked as a field engineer (sales, post-sales) for similar products? Knowledge of the domain can be acquired over time, but the candidate definitely has to have a basic aptitude for technical problem solving and not be scared of getting their hands dirty in touching the product.

One way to evaluate this is to have them demo a technical product that they have familiarity with or worked on in the past.