Partially true. If you don’t close your incognito window, the third party cookies stick around, but they’re not associated with your other profile(s). But, as soon as you do close your incognito window, they’re gone.

Your ISP gets the traffic, there’s no way around that. But, if you’re using SSL (and virtually all websites now use SSL) the ISP doesn’t see the contents of the pages, just the hostnames.

Google gets some of the traffic too, but they pinky swear not to associate it with your other account. I’m pretty sure there would be an employee revolt inside Google if they just ignored the incognito window and associated your incognito data with your main account. It wouldn’t be a technical challenge to collect your incognito data and know what account it belongs with, but if they ever did that and it leaked it would be a huge scandal. Google is so profitable that they have no need to risk that.

Also, Incognito only partially protects you against people with whom you share the computer. Any files you download will still be there. Also, if you leave the incognito window open, it’s there for whoever else uses your computer.