That was too late for any dynamo to be active, but it was also too late for sources like the churning gas and dust of the still - forming solar system to be responsible for the 2 - to 12 - microteslas field identified in the meteorite. (sciencemag.org)
(Earth's magnetic field ranges up to 65 microteslas at the surface.) (sciencemag.org)
The analysis shows that around the year 1370, field strength was falling by 0.054 microteslas a year — substantially faster than today's drop of 0.036 microteslas. (newscientist.com)