I assume its like the term slashdotted which is from slashdot, in this case the headline baits you into some companies advertisement article intended to make you buy into their product.

I wasnt a heavy Slashdot user I read it nearly daily until I found HN.

> What is the Slashvertisement?

I assume that it is a portmanteu of "Slashdot" (https://slashdot.org/) and "advertisement".

Post a technical article highlighting problems with X, and as part of it, mention your product which happens to not have those problems or deals with it a certain way.

So a blog post with the author advertising a product.

A targeted form of unsolicited advertising (ie. spam).