I’d agree with that.
Think about whether the forum is always going to be the main focus of the site or not.
If so put it in the root, then for example you could at some stage have a blog that goes in /blog/
But if maybe your main site focus might be a shop or tutorials/articles then it may be best to have that in root and put xenforo in a sub folder in which case I’d do it now to avoid future redirections.
But if you decide to do that I’d at least have a landing page in the root.