"How "cheap" a security is cannot be defined by pure price?"
That's true. Ridiculous to say one is "cheaper" because the price is lower.
"I suspect LINE is trading at a premium due to a higher yield."
But this is just as silly as the earlier statement. Yield is a function of price (ie. payout divided by price) so to say that price is high due to the yield is nonsense.
Line has a higher yield, but that is because it is an E&P which have higher risk than midstreams. Which is not to say that EPD has no commodity price risk, they do.