Can buried gas pipe conduct flame between two structures?
The possibility of the natural gas line transmitting the fire is extremely close to zero.  Yes natural gas requires air, about 20% mixture of gas and air (more air than gas).  Getting that mixture in a closed pipe, along the entire pipe, would near impossible without some pre-mixing first before the fire.

Also if fires could be transmitted via the gas pipe lines, entire cities would be at risk if just one house caught fire.