So I guess my device is basically fine right ?
Right... these ads are a scourge upon the world. They are so invasive that they do a wonderful job of convincing people that they have a 'virus' on their phone.
First... Android really can't get a virus, in the traditional sense. The OS is just not designed to run the same as a desktop running Windows so its really not capable of getting infected in the same fashion, nor can it really spread them the same way. Now... malicious apps, on the other hand, do exist. For those most part, those require that the user install a bad apk that exploits some security hole to do its dirty deeds.
Here's the thing... most every one of those malware apps usually target known security holes. Because they know that the vast majority of phones in use are running old, vulnerable versions of Android with old, vulnerable security patch levels.
You do not have such a phone. You are most likely on the Dec 5th 8.1 update, so there really isn't any old security hole there to exploit. I can't really recall any malware apps that targeted zero day exploits... it's always one that's going after phones running years old software (of which there are millions upon millions).
But that's not here nor there... think about it... you go on to a site and all of a sudden you get pushed to a page that is screaming in your fact that your phone is infected. It might get cute and pull the model info out of the user string to make it look like it ran some sort of scan. It won't let you leave the page easily and keeps loading you back to that "YOU'RE INFECTED!!!!" page.
Think of it like this... you're sitting on your couch, watching TV and you hear the doorbell ring. You get up and open the door and there's a dude standing there. He immediately starts screaming in your face that your house is on fire and the only way to put it out is to buy a fire extinguisher from him. You look around and its obvious that there is no flames about and as you politely close your door he pushes it open again and starts screaming about a fire.
What do you do? Do you buy whatever crap he is selling to shut him up? Or do you tell him to pound sand and slam the door in his face?