Yes, it's true.
I'm booked on two Easyjet flights to MAD on Thursday - one that leaves LTN very early in the morning, the other leaving at around 6pm-ish. Won't go into why I booked two flights to Madrid on the same day, but the lady from Easyjet told me I could claim back airport tax from whichever flight I didn't use. No refund for the fare paid, just a tax rebate.
It necessitates my having to email Easyjet with dates/times, but for 5 pounds, it's just not worth the hassle in my case.