I feel this is pretty true, but Intentionally feeding is pretty broad. Riot has posted threads about reporting people along the lines of what is acceptable in league, and it's actually appropriate to the community in each region for what is deemed appropriate. There's some broad things like racism or sexual comments not being tolerated, but Riot has told us that according to their reports, each region of the world has different lines for offensive. For example here in NA it's offensive to call someone "bronze", but in the eastern regions of the world like Asia, it's offensive to even call someone Silver. How about that.
This is why I believe offense can be justified under the same category while not meaning the same thing from person to person. In one person's eyes, they may report someone for running down a lane and dancing under tier 1 tower. They believe that's feeding.
I believe something like running into a ranked match with a champion you've never tested, even in norms, or any off meta choice, and doing poorly alongside this, is intentional feeding. You aren't playing smart, and you're holding back on purpose. This is bound to give the other team an advantage, and it's primarily their fault for their not ignorant, but simply stupid choices. They knew what would happen, and they gave the enemy the advantage anyway. I call this fair grounds to report someone for intentionally feeding.
Reporting someone from player to player is in their own eyes to judge. There is a grace to this sort of thing, though. These reports only end up mattering after enough reports have built up to flag their account for investigation. If enough people don't believe it is a punishable offense, it will never happen, or maybe very rarely. If enough people do believe it is a punishable offense, it just might be right to report that guy who's constantly having a bad game, because enough of the community agrees. Fortunately though, I feel like around low silver you can identify when someone is just tilted and not having a good day. I don't feel enough of the community will report these people for it to make any real difference, so try not to worry about these people.