How are these two user drops different?
They are identical.

The bullet point you mention states:


  The user name and host name parts, if quoted, must be quoted
  separately. That is, write 'me'@'localhost', not 'me@localhost'; the
  latter is interpreted as 'me@localhost'@'%'.


And that is what you did, you quoted them separately. In other words, all these are equivalent:

drop user    user_01@host01;

drop user   user_01 @ host01 ;

drop user  'user_01'@'host01';

drop user 'user_01' @ 'host01' ;




And if you wrote:

drop user 'user_01@host01';


... it would be interpreted as:

drop user 'user_01@host01' @ '%';