In this article it later says:
"For example, in the work of Van Soest's group10 11 in which healthy
volunteers were fed controlled diets with the addition of cellulose from
either cabbage, bran, or
a purified cellulose (Solka Floc), average cellulose
digestibility was 74% on the control diet, 75% in the cabbage, about 53%
in the bran but only 25% from the Solka Floc."
This is to illustrate how naturaly sourced cellulose is way more digestable than a chemicaly isolated one. So the answer is that cellulose found in vegetables is 53%-75% broken down. The article also mentions that it seem to vary between people.
The Human digestion system is very much similar to this of primates. In fact, a noticable difference is that the human gut is actually relatively longer. Primates feed on huge amounts of cellulose. It seems obvious that it is energetically beneficial for them. An adult male gorilla will eat up to 30kg of plant matter a day!
So yes, without independetly breaking this stuff down, we seem to do fine with a little help from our friends. Their big contribution is not surprising considering the fact that more than 80% of the cells in the human body does not carry his genes (most commonly because it is bacteria, but sometimes because it is fungi). Methane btw, is just one of three common byproducts of the bacteria breaking cellulose down. That is why cattle growing has a severe adverse effect on atmospheric ch4 (extremely potent green house gass).
Animals can digest the cellulose. Most animals (incl. humans) cannot. The one who can are only thanks to this symbiotic relationship with a anaerobic bacteria that you describe. $\endgroup$