I don't think Sunbrella guarantees to be waterproof at all. Even when new, the bottom side of my Sunbrella bimini would wet a paper towel held against it after rain. It won't pour through or drip, but it isn't 100% waterproof. The guarantee is against fading.
I'm on my second set of Sunbrella covers and they hold up pretty well in the tropical sun and heat - but the stitching used on both systems is becoming brittle and losing tension, I need to be careful when zippering up the bimini or dodger to not break the threads. I would be certain that the shop making your canvas uses premium thread/twine and it pays to have everything double-stitched as well.
The canvas on my old boat lasted 6 years in the tropics, removed during the hurricane season and then kept out for the rest of the year. By that time it would drip through after rainshowers and instead of being a closed cloth material you could see light through it. Both the material and cloth had become brittle. I could have applied some 303 fabric guard to make it last one more season, but it was pretty much EOL (end-of-life).
Sunbrella: Marine tops and covers care & cleaning
All of those I've met with canvas problems have complained about stitching rather than about the base material.