How do we know that clicking the "Login with LinkedIn" button doesn't create the account? OpenID Connect/oAuth logins all work the same - if you haven't already signed up, "logging in as <x>" creates the account for you.
To answer you, the account was associated with my real email but had no password. Saw after deleting the account because it sent a deletion confirmation to my real personal email (that it should never have had in the first place).
To answer the parent post, OAuth doesn't automatically associate accounts. The first time you login (signup) to something with linkedin/google/facebook it gives you a popup warning that you're about to authenticate and share data with a third party service, please confirm.
If you see that popup on login, you don't have an account, don't proceed.
The email makes it sound like you'd need to already have an account at both LinkedIn and SlideShare to have the latter import data from the former.
I also tried their Forgot Password with my LinkedIn email, and it said there is no account associated with it.