I find this surprising, particularly as the point of chewing gum that contains xylitol is to disrupt oral bacteria. Essentially, they try to metabolize it like glucose, waste energy (similar to the way it kills most non-human animals). I use toothpaste that has it as disrupting carious bacteria IMO is a positive but I skip the gum as I'd assumed it would cause the same kind of disruption.

I looked through PubMed (my go-to for this kind of thing). Most studies up to a couple of years ago were inconclusive. But then there’s Effects of Sweeteners on the Gut Microbiota: A Review of Experimental Studies and Clinical Trials (2019)

So far, only saccharin and sucralose (NNSs) and stevia (NS) change the composition of the gut microbiota. By definition, a prebiotic is a nondigestible food ingredient, but some polyols can be absorbed, at least partially, in the small intestine by passive diffusion: however, a number of them, such as isomaltose, maltitol, lactitol, and xylitol, can reach the large bowel and increase the numbers of bifidobacteria in humans. Further research on the effects of sweeteners on the composition of the human gut microbiome is necessary.

For anyone else plowing through my thinking ‘aloud’, bifidobacteria are good, but the “further research” has me wondering how well this might be understood.