The flared end, I assume, is copper which is a soft metal and should seal with no problem. The reason for the warning against using a sealant is, as you have surmised, to prevent the sealant from getting in the orifice and other places blocking the flow of the gas. Before you apply sealant you might want to do a soap bubble test. Go ahead and hook it up and turn the gas on, take a cup with a generous amount of dishwashing liquid, add warm water and mix it up really good then spray or use a small paint brush to really slop it on the fitting and threaded area really good. You will have some bubbles from the soapy water..but look for new bubbles forming... if you can get it on without any bubbles .. that's great..then look for bubbles. No bubbles = no leak..and you're in good shape.