It's a bit of a tricky comparison. Bacterial genomes are jam-packed full of protein-coding genes, while human genomes have genes spread out between much larger regions of both "junk" (viral elements, pseudogenes, and repetitive elements) and regulatory regions.
A human genome is 3 billion base pairs, and a bacterial genome is ~3 million base pairs. Considering there are probably 100-1000 bacterial species of clinical / gut ecological importance in a person's gut, it happens to sort of work out that it takes roughly the same amount of sequencing to sequence your gut microbiome as it does your genome.
Studies like this one do not do that. They only sequence marker genes which describe which bacteria are there (however, this process doesn't include all bacteria, doesn't tell you what those bacteria do, or what their small differences are). It's like a census where you just ask for people's last names.
That being said, after reading I think this was a nice little study. It had a small sample size but pretty clear effects that fit in with a lot of priors that we already have about the gut with regards to butyrate production by specific species.