I agree that plant-based fibre is not the only thing the gut microbes can feed on, and can feed on nutrients from carnoivore diet.
But, if we are feeding the microbes just one type of source, are we not loosing on the fibre-fermenting microbes (and the benefeits attached to them like SCFAs) and affecting the microbial diversity due to lack of cross-feeding mechanism.
Also, I agree that carnivore diet did make humans survive for decades without any fibre but our microbiota has changed from what was it used to be --- some microbial members being completely lost in population of affluent countries.
I myself follow a keto diet and an intermittant fasting but also make sure I get my dietary fibres from plant-based diet to feed the fibre-fermeneters to get the benefits of metabolites produced.
I believe in diversity whether it is about microbes or the food type.