@User thank you for the clarifications. Yes, the nf parameter is relevant for BSIM4, but to be effectively propagated down from the subckt it must be assigned to the mos instance line:

m1 d1 g s b sky130_fd_pr__nfet_20v0_zvt__base  w=w_n20zvtvhv1 l=hvnel_n20zvtvhv1 ad=0 as=0 pd=0 ps=0 nrd=nrd_n20zvtvhv1 nrs=nrs_n20zvtvhv1 delvto=delvto_n20zvtvhv1 m=m

but since this device includes as separate elements the LDD resistance and separate D/S diodes (note AD, AS,PD,PS set to 0) , may be the layout is contrained to a single finger / single geometry? Another concern i have with this device is the multiplication (m) parameter: it is used at subckt level , then assigned down to mos instance (m=m). Isnt't this a m^2 multiplication?