Doesn't sound like we disagree that much. You are actually available, but not all things are urgent. I'd say the difference at my workplace is that pieces of the product are extremely old, developed under different architectures with different technologies under multiple management regimes of varying quality. There's a lot of tribal knowledge, so everything is kind of urgent by default as someone new to a specific piece of code can't move forward efficiently without picking the relevant SME's brain. Forbidding interruptions would be akin to mandating "only the SMEs can work, and the rest of you need to waste money twiddling your thumbs or banging your head against obscure legacy code until they feel like helping you". Most of the time when I'm interrupted it's because someone's blocked and I have the knowledge to unblock them, or I can at least point them to who does, so my loss of flow state is their gaining the ability to move forward at all, and everyone benefits.

By contrast it sounds like you guys designed your product around maximizing the impact of flow states and minimizing the need for interruption. I bet you have good documentation too. I'm honestly jealous, it would take tens of millions and years of re-writes to do the same here, and our prime customer wouldn't be willing to accept the delay even if we had the funding.