Besides, BOOTSZ does not matter if BOOTRST is disabled.
As I suspected but when I looked at an online fuse calculator I expected the BOOTSZ options to be 'greyed out' when BOOTRST is disabled; but it wasn't.
Assuming that the calculator may have a bug in it I tried a different calculator. Sames result.
Having tried a few calculators, written by people with more knowledge than myself, I started to doubt my premise (outlined in your quote above).
Thats when I made this post. An easy conclusion to come to for a relative noob like me.