There's no "1,500 hour rule." There never was. It does not exist.

There is only a requirement for an ATP, and then only for operations that require the ATP. The ATP may be granted with varying combinations of total flight time and education/experience. While the basic requirement is for 1,500 hours, the ATP may be had with less, with some restriction.

We don't know anything about the CFI's "lack of ability," or of his capability. Evidently he flew into convective weather and lost control of the aircraft. In a former life, one of my jobs was atmospheric research: thunderstorms, specifically; I flew back and forth through them with sensors and various equipment. My biggest takeaway from having done that is that while it was interesting and I saw and experienced things that 99.9% of us will rightfully never experience, or hope to experience; my one takeaway is that I don't want to every fly through a thunderstorm again. Regardless of one's capability, there are forces inside thunderstorms, as we're all at least academically aware, that exceed the capability of both man and machine (regardless of ability or capability). Greater skill or experience or the pool of both that makes up a given airman, the greater his or her opportunity to survive an encounter, but forces within separate man from machine, and machine from itself.

Did the CFI's use of snapchat have anything to do with his flying into convective weather? This, we don't know. Certainly judgement played a role. Many of us might have postponed the flight or gone a different direction, and made contingency plans to delay the return, divert, etc.

The application of skills as an airman or instructor on the day of the practical test do not necessarily equate to the product of decision making in the evolution of weather, flight, etc. It's very possible that the CFI said and did all the right things to be blessed with wet ink and the classic admonition that it's a license to learn. Some lessons don't end well, particularly for the student who doesn't pay adequate attention.