Why would ATMs need GPS for accurate timing?
They don't. Article is wrong.
There is a strong trend recently to install GPS-based trackers inside the unit, and if they start moving, call the cops on the included cellphone.
Assuming you're not using the jammer to steal the unit, the only other use for jamming the GPS, is as a denial of service.
Actually, if you read the article, they do need accurate timing:
"And ATMs sometimes communicate wirelessly, using a time-based encrypted code that requires synchronisation."
Using a cryptographically secure PRNG seeded from a known time as an authentication token is a well-known, and frequently used technique. For example, many of the "keyfob" type tokens such as the RSA SecureID things. (Which didn't use GPS, and instead used an internal clock and server-based skew tracking if I recall correctly, but the the principle is the same.) I don't know enough about ATMs to weigh in on what schemes they actually use for authentication and encryption, but it certainly is plausible some could use a time-based, GPS-driven scheme. And, as a bonus, you then have a GPS in the machine for tracking.