Yeah, that is true. Mainly because the deletion of its supercharger for production, but also relevant to the topic here, pilots didn't love having that drive shaft between their feet.
Still a functional design though, with certain advantages.
Who the hell was responsible for all the supercharger deletions? If this was done by a bureaucrat, then this is a prime example of tremendous damage done by bureaucrats to their own side in wartime. P-38. P-39. I'm sure there are other craft that were effectively nerfed.
I don't think that history is well understood, and there are a lot of urban legends around it, I think. It may have been a political/bureaucratic thing. It may be that Bell needed to stop fiddling around with design and start manufacturing airplanes before they went bankrupt. It may be that we wanted a second-tier fighter to give to some of our less friendly allies, that we could beat if we had to. It may be that Bell just couldn't get it to work right. Another story that I have heard is that they were fearful of losing the supercharger hi-tech to enemy hands. I don't know which, or to what degree any of those stories are true.
The juiced up v2, the P-63 King Cobra had a proper supercharger system. It was more capable at high altitude - though the USAAF felt that the P-51 was better.
After the war, the remaining few P-63s (and P-39s that could be retrofitted with better engines) dominated air racing. Flying very fast at low altitude and turning on a dime (that mid-engine) generally out flew the P-51s. But since most of them were sent to the Soviets and elsewhere, and our own leftovers were simply scrapped or used for gunnery drones, there are very few left. I have never seen one in person.
Still a functional design though, with certain advantages.