What does it mean by "fitted to a normal distribution and a threshold of p < 0.001"?

Question

[Simon Ahn]

According to the paper, it says

I have data look like this

I used G1S score

library(fitdistrplus) 
library(dplyr)

GBM_meta_data <- read.csv(data) G1S <- GBM_meta_data$G1S_score 

G1S <- fitdist(G1S, "norm") 

plot(G1S)

and I got result like this

However, p < 0.001 means that G1S score should be higher than around 3, I think. However, when I looked at the paper figure, it seems they defined cycling cells when cell has G1S score more than around 0.8

I have no idea how to calculate right threshold for G1S.

Can please help me with this?