Wikipedia agrees by mentioning an early eight-shaped roller coaster:
Die erste regelrechte Achterbahn (in Form der Ziffer 8) wurde 1898 auf Coney Island eröffnet.
There were roller coasters before that one, but the form was popular at the time.
Duden also agrees by implicitly mentioning it:
(auf Jahrmärkten, Volksfesten o. Ä.) mit großer Geschwindigkeit auf- und abwärtsfahrende Bahn mit Kurven, die zum Teil die Form einer Acht (a) haben
It also has an entry for "Achter":
Figur in Form einer Acht (a)
The Grimm has several entries for "Achter", but I couldn't make a connection to the "Achter" in roller coaster.
Since there's no other explanation, several dictionaries / websites agree and there actually were a lot of 8-shaped tracks in the beginning, I think you can be fairly confident to believe this to be the reason for the word.