Nope, it is not considered advertising.
Summary
The reason Moderators (Of the human variety) exist is to be able to make a well informed judgment call.
Using the forum as a platform for advertising is not permitted under the Forum Policy.
However, sharing relevant information to help other users find products that they need is permitted.
Affiliation and Intent are large factors.
On a technical level; the share your desktop thread would contain a great deal of advertising if we viewed it in such a strict manner. But we do not... because sharing information about a theme or gnome extension or GnuLinux-friendly product is an act of helpfulness and community.
A company affiliate posting links to their own products for profit all over the web is not a part of that community.