I think you're understating it. Yes, any intelligent storyteller will have a plan loose and adaptable enough to be adjusted over time. But that doesn't mean there's no definite plan at all. It just means that plans are meant to be frameworks, not prison bars. It means that flexibility and change are integral, normal parts of the creative process. You have a definite idea of the big picture, the key points you need to hit, but you allow yourself leeway to adjust the specifics and rearrange the pieces, so you're able to adapt to changing circumstances or rework when you have a better idea.
Those of us who were first-generation Babylon 5 fans saw this happen with J. Michael Straczynski's master plan. He had a whole road map plotted out for a 5-year series, but he was frequently forced to make adjustments due to cast changes and unexpected circumstances. Yet he was able to rework his idea and incorporate them in different ways, which were often an improvement. When the show wasn't renewed for a fifth season, he compressed his last two seasons' plans to tell the whole story in one season. Then TNT picked it up for another year, and he rebuilt a fifth season by combining leftover pieces of his original plan with elements from a sequel series he'd intended to do. So he managed to work in most of the concepts he'd intended to cover from the start, even if they ended up being done in different ways than he'd originally envisioned.
No, of course the MCU doesn't have detailed scripts written yet, because that's the purview of the individual creators they hire and they're right to give those creators the freedom to do their own thing. But that doesn't mean they have "nothing more" than the vague generalities they've allowed us to hear about. I'm sure that Feige and his team "break" their phases the same way a TV writers' room breaks its season arcs. They work out a list of the key story and character beats that have to happen over the course of the season (or phase), and then they assign specific writers to work out the details of what parts of that list go into which episode, and how they're incorporated. It's probably all on a big whiteboard somewhere so they can erase and rewrite as they go. There's usually a whiteboard.
I've had similar experiences when I've worked on collaborative Star Trek prose projects. I've been assigned to tell a particular part of the story and work in specific beats, but it's been left to me to work out my own story and decide how to incorporate those beats and concepts. For instance, in Greater than the Sum, I was told "wrap up the loose Borg thread from the last book" and "have Picard and Crusher decide to start a family," but it was entirely up to me how to do those things. Still, it was known what the status quo would be at the end of my book. That was already locked in ahead of time. A loose plan does not mean there is no plan.
