Python managed to do this by not actually checking the types at runtime. If you declare a list[int] return type but you return a list[string] then nothing happens, you're expected to prevent that by running an offline typechecker.

PHP chose to check types at runtime. To check that a value is really an array<int> the runtime could have to loop through the entire array. All the types PHP currently implements are simple and cheap to check. For more elaborate cases you need an offline checker like PHPstan and comment-based type annotations. (PHPstan catches 99% of issues before the runtime gets to it so for my own code I'd prefer the Python approach with its cleaner syntax.)

The runtime checking seems the key difference, not so much the historical strength of the type system. Python's language implementation does very little typechecking itself and PHP's third-party offline typecheckers are respectably advanced.