You can fly for 30 minutes after sunset.
You should have a light visible for 3 miles for flying in the half-hour after official sunset, though.
LED strobes are available that would suit.
103.11 Daylight operations.
(a) No person may operate an ultralight vehicle except between the hours of sunrise and sunset.
(b) Notwithstanding paragraph (a) of this section, ultralight vehicles may be operated during the twilight periods 30 minutes before official sunrise and 30 minutes after official sunset or, in Alaska, during the period of civil twilight as defined in the Air Almanac, if:
(1) The vehicle is equipped with an operating anticollision light visible for at least 3 statute miles; and
(2) All operations are conducted in uncontrolled airspace.
Assuming for plausible deniabiltity that they had an anticollision light and were going to be top landing, they had 13 minutes of legal flight available.