How do you think they're able to post record quarter after record quarter? It's because their singular competitor hasn't been competitive. Their high stock price is predicated on AMD remaining noncompetitive, or on the automotive side of the company growing to the point where it can pick up the slack if/when they have strong competition again in their primary market.
The fact that Nvidia has been able to post record quarters requires more than just a lack of competition. Take Intel for instance, they are in pretty much the same position as nvidia competition wise and yet they haven't been able to post result even close to what Nvidia is doing (growth wise), so clearly this is not simple a result of a lack of competition.
That would be a good reason, but the fall in stock price seems directly related to the downgrade, and the reason for the downgrade is: "We believe consensus is underappreciating a slowdown in gaming and the potential negative impact to the multiple.
If you look at Nvidia's historical P/E ratio, you can see that the correction has actually been underway for some time now (since december 2016), so clearly that can't be a result of the downgrade that was announced last thursday, also the stock has been going down since the beginning of february, so again clearly not directly linked to the downgrade that came later. The large 10% drop would likely have been a direct overreaction to the downgrade (overreaction since the stock rebounded by 5% later on), but it is part of a longer trend that started before the downgrade. As such the reasoning given would actually seem to be apt, and notably the reasoning has nothing to do with competition from AMD.
So all in all the stock became overvalued because investors overestimated the health of the gaming market and the speed with which Nvidia would be able to transition away from it to deep learning and automotive (and thus overestimated Nvidia's future growth potential), not because investors underestimated competition.