You are right to be skeptical here. Brake lines are just rubber hydraulic lines and fail in "normal ways" frequently. Without ANY forensic details on "the cut" it is very likely coincidence going viral "because Elon"
How the actual fuck do you coincidentally end up with failing brakes right after a controversial social media post? The braking system is designed to be one of the most resilient systems in a vehicle. A coincident like that sounds a cosmic ray bitflip level of rare.
Among crashes where a "vehicle defect" is identified as the critical reason for a crash, brake problems account for 22% of them. It is a normal failure.
Both of these things can be true. Brakes can be the most resilient component in a car, but since they are so important, failure usually guarantees a crash.
It's nuanced. The brake system has several failure modes. Dual reservoir master cylinders would result in only half of the system to fail. You would absolutely be able to stop a car using only front or only rear brakes under normal conditions.