This is a ridiculous argument from both logical and practical standpoints. The basic reasoning is that if one can conceive of any situation where the military would be unbeatable, then the military would be unbeatable in all situations. This is a very poorly reasoned non sequitur.
There are basic assumptions that these contrived scenarios make that simply cannot be relied upon. One of the improper assumptions is that such a confrontation would happen as open battlefield combat. This is absurd; no tactician would suggest a course of action with the least probability of success. If we've learned anything from our experiences in Vietnam, Iraq, and Afghanistan, it's that the military successes against a native population employing guerrilla tactics are not such a cakewalk.
Another bogus assumption in this hypothetical situation is that 100% of the military would be on the side of this tyrannical government. Clearly we cannot rule out that the military might divide into factions, and perhaps even the largest faction would side with the people's militia. Such a confrontation would make it much more difficult to predict the outcome.
The point of the Second Amendment in all of this is that regardless of the outcome, the probability that the people would prevail is much greater when the people are better armed. It also should be stated that in a remarkable feat of double-think, many of same the people making the above argument are also the people attempting to ban ownership of those weapons that would be most effective in this scenario. If they really believed what they're saying, that's an argument for making more powerful weapons available to average citizens, not restricting them.