Igneous rocks are formed by molten rock solidifying. They seldom contain fossils because the heat tends to destroy the remains. You might have been thinking sedimentary rocks?
As I understand it, radiometric dating of rocks is based on when the rock solidified from lava. According to what I have read on the subject here, sedimentary rock would not necessarily be the measured age, unless we some how knew it was eroded and then formed soon after the rock solidified.
My impression of the typical case is that dated fossils are found in some layer of sedimentation sandwiched between layers of igneous rock. Then radiometric dating is performed on the igneous rock layers and the date is interpolated across the layers--but I don't have even the slightest practical experience with such things...and I wonder how I can be certain the dates in the article's case are really accurate or if the researcher in this particular case is a bit of a quack.
Some igneous rocks are volcanic ash, often call tuff, which might be relatively cool when they reach the ground. Also fossils can be found in non igneous layers between igneous layers. For fossils/remains older than 50,000 years there are other means of measuring than C14. Hopefully some other FReeper can tell you about them or you could Google “dating old geologic material.”