Ok, so you have a review article about PC networking equipment, sponsored by Cisco. That's an ad for you. Now you go to the homepage of the site that wrote that review. They link to that review from their home page. Is that an ad? Probably, because it could contain an ad-like image as its banner (showing Cisco equipment) and maybe even a slightly ad-ish title ("Cisco outperforms other networking equipment in our recent review of x articles").

So, is another page, that links to that page also an ad? Because maybe the homepage that runs Cisco-sponsored reviews might also have a cisco-sponsored banner. And even if not, maybe over time they become associated as "the thing that reviews things were cisco always wins"...

Oh, and what if it wasn't a sponsorship. What if instead Cisco just gave them the hardware to test at a discount? Or for free? Would you not write a slightly more favourable review for nvidia if they gifted you their top-tier GPU? What if instead of gifting you the GPU, they just give it to you at a discount? Or earlier than normal customers? Or they just have nvidia-stickers for your kids in the box?

Ads are an extremly slipperly slope, they're part of human nature; everyone I ever met is trying to tell me about (=market) themselves, their beliefs, their current hobbies, that new burger place down the road, ....