Sorry but this is nonsense. Juniors can absolutely make a huge mess in python if let to their own devices. You seem to confuse the knowledge of syntax and language features with actual experience in designing software.
If we're just talking about the python stack arguably there's not much else on top of it. You get the extra stuff through osmosis in the same 1 - 2 years. The applications for python aren't wide ranging enough to include much else unless you're thinking about data science and ML stuff.
I guess it depends on your definition of "senior", but I don't see why language choice should affect it this much, given that much of what makes seniors senior is not knowledge of specific tech, but experience with software engineering as a process.
