Re: Fortinet is blocking sites even though I never installed it.
So, I don't know if this is exactly what the OP is referring to or not, but I have seen Chrome give this message (sometimes) when attempting to connect to a web site that is blocked via a Fortigate firewall web filter policy. I've attached a screenshot of this (this was from an attempt to connect to Reddit in Chrome while Reddit is blocked via a URL filter).
OP, if this message looks like the one you see, then my guess is that it's just an erroneous reporting from the browser as to what is actually happening. If you are behind a Fortigate firewall, and the website you're attempting to visit is blocked (and maybe even more specifically blocked via a URL filter vs their content filter or something), then it is possible to see this message.