Why FT8 is an 8-tone FSK mode with 6.25 Hz spacing between tones and a symbol rate of 6.25 baud. Eight tones spaced 6.25 Hz apart occupy 8 x 6.25 = 50 Hz, so each FT8 signal takes up only about 50 Hz of spectrum. That narrowness is why dozens of FT8 signals can share one SSB-wide slice of a band.
Watch out The 2.4 kHz figure is the width of the SSB receiver passband used to hear FT8, not the width of a single FT8 signal; many individual 50 Hz signals fit inside it.
8 tones x 6.25 Hz spacing = 50 Hz. FT8: eight tones, fifty hertz.