I'm not saying that she wasn't stoned off her ass... But the fact that she was able to divine answers that were reliable (at least far more often than not)... Even if it wasn't 100% accurate, it was still more than chance would say that she could get right.
Lets face it, you can smoke/drink/eat/snort almost anything, get ****ed up, and start trying to tell the future... But what percent of what would come out your mouth would be right? But at Delphi, she was right enough that she was considered reliable...
The old idea of gas seepage is not well accepted by most academics and is mostly entertaining fodder for TV specials (like in the good old days when the History Channel used to have programs about history instead of redneck swamp dwellers).
But the oracular divinations preserved in ancient writings reveal prophecies so vague as to be generic. All of them could be interpreted in numerous ways. Let's review a couple of the most famous.
Croesus and the Persians
Croesus, king of Lydia, went to Delphi to see if he should take on the advancing Persians. His oracle: "If you cross the Halys River, a great empire will be destroyed." Naturally Croesus believed it meant that he would destroy Persia, but the story works great for the Greek mind because they were all about hubris. As it turned out, it was Croesus' empire that was destroyed.
Athens and the Persians
As the Persians under Xerxes were approaching Greece, an Athenian delegation went to Delphi to ask for advice. Their oracle: "Athens will be saved by her wooden walls." Some felt this meant the wooden palisade on the Acropolis, so numerous Athenians remained in the city and took up arms on the Acropolis as Persia invaded the city. Themistocles argued that it was a reference to Athens' great naval fleet, and most sided with him. The Athenians on the Acropolis were slaughtered, while the fleet under Themistocles defeated the Persian navy and sent Xerxes packing.
In both cases, the divinations could be taken more than one way. They were sure to please at least a lot of people—who only wanted to be told what they wanted to hear. It's human nature. In all probability, the priests of Delphi who "interpreted" the mumble-talk of the Pythia in "commune" with Apollo, were hedging their bets and striving to remain as generic as possible. They needed to earn a living, after all.