This is a bad rule of thumb in some climates. Your technique of piling on insulation might create a dew point at a place that isn't ventilated, and thus cause a buildup of mold.
AFAIK, cellulose insulation is chemically treated to be mold-resistant and fire-resistant. Also, remember a rule of thumb is just that: by definition it can't cover 100% of edge cases. If you have a water leak in your roof, you obviously have bigger problems to address first...
It doesn't matter much if your insulation is mold-resistant. Presumably it's resting against wooden beams or other things that would be friendly to mold, or the water can drip down once it condenses to something that's friendlier to mold.
You wouldn't get water from a leak, you'd get it from thin air via condensation. Ever had fog on your car window? That's water on the inside of your window, and not because the roof of your car has a hole in it that's letting water through. Water condenses out of the air.
Making sure condensation happens in the right places is one of the main jobs of well designed insulation. What you saw as shoddy work from a contractor may have been a standard insulation design that's intentionally skimpy on the internal insulation, because the condensation should happen in an outside layer.
By piling on insulation you may have moved that dew point to the inside of the house or past a layer of insulation meant to deal with it.
The rule of thumb is that you can't just add more to one layer of your overall insulation (which is everything from your outside weather layer to the paint on your inside wall) without considering the system as a whole.
Search for "hygrothermal analysis" or "dew point" in relation to outer wall insulation. You'll find resources like [1] and [2]. The former has an example where more interior without corresponding changes to the exterior makes things worse.
I'm no expert on insulation, just a fellow home DIY-er. I've had enough experience with it to be very paranoid if I were in your shoes. It's worth bringing in a professional to evaluate this home improvement job of yours,
You may have created conditions for mold to thrive in your attic, or for water to seep into structural layers of your roof. By the time you notice those sorts of problems without having to open up a wall it's often too late.
I did have a mold issue in the bathroom shortly after buying my house (a few years before adding insulation). Had to redo the ceiling paint job, which was annoying to be sure, but that was about the extent of the damage, and we never got a mold problem again afterwards.
I also did have someone come over to quote me on insulation work when I was planning it (since the mold thing was a concern). They said my setup was fine as far as mold risk was concerned, and that it was clear that the two layers that were there were insufficient.
If in doubt, you can always ask for a professional opinion before attempting any DYI.
