potentially polymorphic call with model instance inside model function

I'm porting a CI3 app to CI4 and need to call one model method inside another model method.  Can someone tell me why this produces no warning:

PHP Code:

public function get_value(): int
 {
 $this->sale = model('Sale');
 return $this->sale->get_quote_number_for_year();
 }
} 


But if I do this:

PHP Code:

public function get_value(): int
 {
 $sale = model('Sale');
 return $sale->get_quote_number_for_year();
 }
} 


I get a warning from PhpStorm
Potentially polymorphic call. Model does not have members in its hierarchy

The function signature for get_quote_number_for_year() is
public function Sale::get_quote_number_for_year($year = '', $start_from = 0) int