Because I was worried about having used the pens to mark a t-shirt quilt I made for DDIL, I decided to do a test before using them again.
Using a discarded sleeve from a t-shirt, I marked on it with a black Frixion pen, then threw it in the wash, cold water, with my regular laundry. It also went through the dryer. It came out as clean as it was before I marked on it. The final test was putting it in the freezer to see if the marks would reappear. They did not.
Based on my admittedly limited test, I now consider them safe to use on a quilt that will be laundered.