Just continuing this theme it does appear that parameter list to the SevSeg begin() method may have changed over the course of time. This goes back 7 years: Display on seven segment display. - #4 by billhowl
The implication is that the option to disable the decimal points came later. However, the user was then expected to always deliver 8 segment pins.
Anyway, I'm coming round to the conclusion that this is best regarded as a user error and the way the library is structured with parameters including 'C' style arrays, there is little that can be checked if the user gives misleading inputs.