Are stars expected to become dimmer before a supernova?
The connection between the dimming and a putative supernova relies on the interpretation that the decrease in luminosity may be due to circumstellar material, ejected in the years/decades/centuries immediately preceding a supernova. There are several mechanisms that could lead to this sort of mass loss (see slides 24-25), including


gravity-wave driven envelope loss (in red supergiants, during neon/oxygen core burning)
pulsations due to pair-instability in the days to decades prior to the supernova
turbulent eruptions à la luminous blue variables


These then lead to Type IIn supernovae, with narrow lines arising from interactions with the previously ejected circumstellar material. If the dimming is due to extinction by circumstellar dust from eruptions due to any of these mechanisms, it could mean that a supernova is imminent on a timescale of days to years.