ReactOS does not steal any of Microsoft's code. In fact, having the leaked Windows source code is as bad as looking at it... and the developers cannot have them or they cannot work on ReactOS. The same discussion occurred with FreeDOS about the leaked MS-DOS sources.
There may be areas where ReactOS will strugle in the same places that Windows NT did or is currently. However, this will not be because of code stealing. If we wanted a better, yet compatibile, operating system than Windows NT, why would we steal the leaked code? That wouldn't make sense.
I'm not a native English person, yet I can speak and type fluent English. It's not that hard.