1999 Honda Civic Check Engine light after Timing belt repla

On the way home from having the timing belt and belts replaced on my 1999 Honda civic the check engine light came on at highway speed. The plugs, plug cables, distrubutor, and rotor were all just replaced. The light does not come on at lower speeds/around town driving. The code comes back that all cylinders are missfiring. The car seems to be running fine. The mechanic (I decided to have them do this job) told me that their light on their meter to check the timing was not working and that they had readjusted the timing. The car still gets the missfire code at highway speeds. The first time the light has ever come on is on the way home from getting the timing belt replaced.

Thank you