By opening open the brake reservoir, maybe also pulling the fluid out from there, i.e keeping it open for some time. Opening the new bottle of brake fluid and pouring it in you have probably introduced more water in the form of humidity than you started with because brake fluid is hygroscopic
Brake fluid - Wikipedia
I worked as a GM mechanic for 10 years and unless we did a brake job including changing out the calipers or caliper seals we were never ever advised to completely flush the fluid and during that time I never heard of a reasonably maintained GM vehicle losing it brakes because it boiled the fluid due to water or degradation of the fluid.
If the manufactures were concerned about not introducing humidity into the system there would be a way to fill the fluid without exposing the reservoir to air. Additionally, the retail price a brake fluid tester is about $7 so the senors would be say a dollar, tell me they could not integrate this sensor into the brake warning system economically if they were so concerned.
This whole brake fluid changing at 30k intervals is costly for the average punter (i.e. paying a dealer to do it!), useless and as I argue above possibly even counter intuitive.