Right. As acknowledged in the article, Xarantaur is an immortal -- but he didn't start off that way. He himself will tell you that he was about to die when granted immortality by the Bronze Dragonflight, which would put him anywhere between 100-150 at the time that that happened if the Warcraft RPG is still canon. But Malfurion Stormrage and any of his contemporaries would have been even older than Xarantaur in this same period -- again, assuming that the Warcraft RPG is accurate.

The average lifespan of a night elf before Nordrassil's immortality was in the region of 2,000 years, and they were considered young adults between 100 to 300 years. Malfurion at the time of the War of the Ancients was said to be a young adult; ergo, he's somewhere between 100 and 300 years old when it happened.

Xarantaur, by his own account, has also lived through the War of the Ancients, the Sundering, and a period spent afterwards traveling Kalimdor collecting stories (which I can only assumed comprised the vast majority of his life as he insists he assembled a fairly comprehensive set of stories from most Kalimdor cultures. Given the size of the Kalimdor landmass, there's no way he did this quickly). He's not gifted immortality until he's about to die at the end of his natural lifespan, which puts him anywhere between 100 and 150 years old then. Which means that he, too, was probably a young adult (or young-ish) when the War of the Ancients occurred -- which by tauren reckoning puts him somewhere between 30 and 50 years.

In other words, when the War hits, Malfurion is between 100 and 300 years old, and Xarantaur is between 30 and 50 years old. Their later immortality is a non-issue, because Malfurion is already older than Xarantaur. Xarantaur may very well outlive the night elves assuming that he retains his immortality and the night elves don't get theirs back, and it's conceivable that he'll be the oldest druid in existence as the more elderly among the night elves succumb to a natural death (or else get killed in battle).

But right now there's no reason to doubt that Blizzard's official line is disputed by Xarantaur's account. Xarantaur is old, but a host of night elf druids (and Malfurion himself) are still older.