It's no rumor. Pine trees do acidify the soil, but a pH of 5 (which is typical) is not a problem as there are many other plants that are happy to thrive (and even require) a more acid soil. Blueberries are one such example.

I didn't commission a study or anything but the first page of results on Google suggested that pine needles are acidic because they prefer to grow in acidic soils but they have no method of acidifying the soil by themselves. Also pine needles can form a dense mat which tends to exclude other species.