Why use category in the url. It would be better if the category name was there
Structure and organization. Most modern forums use this format. Lets give an example here.
... and so on and so forth.
So to answer you question the the category name IS there and it is appropriate to use the category in the URL for the structure and organization of URLs. NodeBB is SEO optimized.
Ok, hmm. I thought that it would be better to have the relevant part in the beginning of the url. I think google would prefer that.
Whether Google would prefer it one way or another, we can only guess. An entire snake oil industry is built around optimising sites for high search rankings, but in the end, the biggest contributor to your ranking is whether or not you have fresh, user-generated content...
Frankly, a forum is hands-down the best way to expose and incentivise user-generated content for indexing. Everything else (meta tags, URL structure, etc) is just icing on the cake.