That's right. You basically won't be able to do a cardio effort to your body's capabilities. Anyone that has ever run an ultra marathon is essentially operating in ketosis like state. You just burn more energy than your body can absorb so it is a combination of stored fat and incoming carbs/fat/protein that keep you moving.
edit: Your body also slows down its nutrient absorption rate during any exercise as it diverts blood to your extremities and core body functions. So digestion is more of a challenge.
Studies that show a drop in cardio performance from low-carb have been measuring in the adaptation period, before the body adapts to low-carb performance.
This is covered pretty thoroughly and with a much higher level of detail in Lore of Running (Noakes). Down to the biochemical level of what is actually going on as you adapt to a low / no-carb state. You have to be precise with what you mean by "long-duration" cardio. Up to Marathon level you simply won't be able to go as fast as a runner that has muscle glycogen. You won't find any examples of elite marathon runners performing at their peak on a no-carb diet. Somewhere right at, or shortly after, marathon distance for most runners you have used up your glycogen stores anyway, so a great portion of your ultra is going to be executed without a lot of carbs / easy glycogen anyway. I don't think anyone will dispute some runners can operate with no carbs, etc. So, it just makes sense to train your body to burn fats / body fat efficiently because that is where a big portion of your fuel comes from in an ultra.
I can't remember the exact time period, but it only takes a few weeks to fully adapt to low/no carb diet where your body gets good at making energy from fat. The actual chemical process is really cool.
You definitely lose your ability to do high performance shorter events at the peak of your body's capability (5K/10K, etc.). It is simply a less efficient process of creating the primary fuel your cells use to do their thing.
You also have to be precise about what you mean by "elite". Definitely "most runners" don't fit that category, so saying "you simply won't be able to go as fast" is probably overgeneralizing.
“If you’re going to run a marathon under two hours and 10 minutes, then yes, you probably need lots of carbohydrate to be able to perform to such an intense level of activity, but once you’re slower than two and a half hours, I’m not really convinced you do.”
Adaptation periods are probably 6 weeks, possibly longer to fully adapt, though adaptation appears progressive.
And training low-carb can have benefits even if racing carb-loaded.
My understanding, barring real genetic outliers, is that the human body just can't go as fast in a fat adapted state. I would say "any runner". Meaning anyone with a few months of real training under their belt is going to run sub marathon efforts slower. The article you linked pointed that out. This isn't just for elites, but everyone. Most recreational/non-professional runners run sub marathon races. This implies some important things about how you train and eat. 1.) When doing max effort running, to push your body to it's limit you need some carbs. 2.) To run your fastest race you need some carbs. You can still train and do well without them, of course. I think for most runners it's fine and the difference doesn't amount to much in practical sense.
Here's an article that talks about the process of fat adaptation. https://medium.com/@davidludwigmd/adapting-to-fat-on-a-low-c... Note the graphs showing ketone levels continuing to rise through 38 days of complete fasting. If you are ingesting some carbohydrates or protein, this would slow the adaptation process beyond that.
There are lots of studies showing a performance decrease during within those early few weeks of the adapatation process, but they aren't really showing the fully adapted, optimized state.
