So from 8pm to 9pm you went and asked all your instructors? Sure you did. I think they'd be embarrassed to see you claim their opinion supports yours. Which magical FTO are you at that believes this?
seriously i'm not going to get petty but I start work at 8am in my FTO, I asked 4 instructors that have more experience than I do (two of which are PPL examinars) what their understanding of FM meant to check that what I believed was correct & all aggreed with my comments. I also got one of them to check in a met textbook for a definitive answer which was;
"The FM indication in a TAF denotes a change in the forecast conditions begining to occur after the time designator"
baring in mind that one of the four instructors I checked with has been teaching groundschool for over 20 years & another has a degree in Met as well as the fact that the PPL textbook states the same as what I thought to be true I think this is conclusive.
& for the record, if the TAF had stated AT 1800 then I would have agreed that the vis would have been 10km+ however FM & AT are two very different things.