Well, #ff0000 is the definition of "red" within the RGB colorspace, corresponding to the full brightness of the "red" (only) pixel components, constrained by your monitor's limited reproduction gamut.
So yeah, it's "red". But it's not the color of anything that has ever been viewed by humans in real life (except itself). There are few pure colors in nature anyway, but you can talk about "red" being wavelength of the center of the "reddish" range of the visible light spectrum.
Of course, names for colors are always subjective. Crayola might be the best universal reference. Crayola red bears little resemblance to #ff0000, even discounting the pigment vs. light mixing and perception differences.
But you have to call it something, right? So #ff0000 is "pure red, RGB, luminous". :)
The hue 5B in the Munsell system is not supposed to be some idealized “blue”. The “B” is just a mnemonic device: breaking the 100 hues which make up the full Munsell hue scale into 10 sections R, YR, Y, GY, G, BG, B, PB, P, and RP is a way of making the numbers a bit easier to remember than just a number from 0–100, but the precise boundaries of those categories are not in themselves meaningful. The whole point of the Munsell system is that you learn what color the hue number represents, and then it’s predictable, you don’t need to rely on someone’s arbitrary choice for each color term, or the specifics of some printer inks. The 100 hues are uniformly spaced with respect to human perception, and since human color categories are not organized in an orderly way relative to hue perception, it’s inevitable that any rigorously organized system won’t match up precisely with those categories.
As for RGB #FF0000, the “R” primary on a computer display is very orange-ish compared to what a human would call the unique hue “red”.
Edit: and the color called "blue" in the image on top of the Munsell system wiki page is not blue, it's some shade of teal.