Hey Tina,
I'm afraid your colleagues saying that the best way to go is to have it under a subfolder are right. Basically, all search engines consider subdomains as separate domains, meaning that if you put the learning center on learning.website.com will have the same effect as having it on somenewdomain.com, neither will help website.com unless you link from one to the other, and that could be seen as doorway pages which will ultimately hurt more than they could help.
Using a subfolder, any advantage you get from website.com/learn/ will influence website.com without putting website.com at risk (unless you post completely unrelated stuff in that new folder). But as you trying to write about the same niche, it is like basically having a blog. Take this Moz section for example. The community folder in a subfolder under their "money site" and I assure you it is working far better than if it was a subdomain.
I'm with your colleagues on this one
Hope that helps!