This also depends on eating frequency. If someone has a very low eating frequency, like a single meal every 72 hours, even a meal of lean meat may not knock a person out of ketosis, at least for very long. I've fasted that long and eaten a huge meal of lean meat, multiple times, and my keto testing strips always remain dark purple.

As much as I think keto and carnivore are lightyears past the average American's diet, I've found them difficult to maintain because they're very restrictive and I lack the energy to be effective when working out, which is common. Fasting in combination with a low carb diet, even one that incorporates things like berries, has been much more effective and easy to maintain in my experience. Combining macronutrition with eating frequency reduces the penalty of eating carbs or lots of protein by making it much easier to get back into ketosis. When you force your body to burn through its glycogen stores, and you give it no choice but to use your fat store, ketosis will come back in no time.

So you are definitely correct that fatty cuts are better, most of the time, but with fasting the protein to fat ratio almost becomes a non-issue.