It's not the fat content of quorn sausages, it's the carb!
When we say to steer clear of something, it's generally when someone's starting out and/or asking for help because they're stalling/losing slowly. These sort of things can of course stall you because of their carb content, but it will depend on the rest of the day's menu too (some tolerated things are carby) so where one person might be fine, another mightn't! I believe quorn sausages are ok as a tolerated.
Good luck and keep an eye on the scales (AND your hunger level - that'll soon tell you if you've slipped out of ketosis!)!