Why can't TypeScript automatically assert types in this example?

Question

interface Model {
  a: string | null;
}

interface ModelA {
  a: string;
}

function myFunc(m: ModelA) {
  console.log(m.a.length);
}

function ModelFactory(): Model {
  return { a: null };
}

const model = ModelFactory();

if (model.a) {
  console.log(model.a.length);

  myFunc({a:"s"});
  myFunc(model); // Errors
}

TypeScript playground link.

I know I can write a user-defined typeguard or use " as ModelA" but both feel like sidestepping the type system. Any reason why it can't use the if type guard and conclude the second call to myFunc is also typesafe?