No one says its absolutely unbeatable, but you have to look at it in context: One could say that after 30 years of services the F-15 has not been shot down in A2A combat. You could say that the F-15 was unbeatable by the aircraft and pilots who faced it throughout those years.
Same situation with the JSF. Given the use of good tactics, a good pilot, and proper electronic support the JSF will be a very effective aircraft, unlikely to be beaten by a MiG-21 (assuming that is what the politician was referring to).
Its not impossible though--in one-on-one combat, all else being equal, the best pilot should prevail. Again, look at the example of Supa Hornet's getting simulated kills on F-22s. On paper it just shouldn't happen.