Different python versions have different syntax grammars, so if the rest of your file has new syntax, and older python might not be able to execute even the first few lines.
For example if you run this on python3.6:
print("hello")
match 123:
case _: pass
What is the problem with that? I see no reason to expect that would work.
The whole point of writing a "self-contained" script is that it should run anywhere. uv bundles its own cpython runtime(s) for this purpose, but relying on script execution prior to invocation of uv breaks this.
The trick in the featured article would allow me to drop a script written in modern python syntax on my outdated ubuntu LTS box and have it "just work", while GP's suggestion would not.