But surely telepathy goes further back than that?
Absolutely. The Vorlons were manipulating the DNA of the younger races millions of years ago, for various reasons. But, again, it was the Human telepaths who were the Vorlon's "pride and joy" (as I believe Lyta put it) and their greatest success. And the Human telepaths of the Psi Corps era were only stage one or two of a multi-stage plan. Sheridan upset the apple cart before they got too far. The Vorlons probably had long-range plans for the next several thousand years and the next two or three conflicts with the Shadows.
Were they the exception to the rule, or did they make themselves that way through science?
I suspect the latter. Given how much Vorlon science is based on truly organic technology (unlike Shadowtech, which is based on machines imitating biology, rather than biology acting as machine) it is hard to say which Vorlon abilities were biologically natural and which were artificial Even the artificial ones could have been grafted onto Vorlon "DNA" eons ago so that the last Vorlons to actually be born would have been "born" with these abilities.