Phased array antennas are mostly for getting programmable beam forming, and don’t have much to do with jamming, which works by destructively interfering with incoming waves. Starlink operating on a different frequency means that GPS jammers won’t be able to jam them out of the box, but presumably if they’re jammers built in the last 20-30 years they’d have onboard SDRs which can dynamically hop frequencies. It’s a neat trick to get GPS from Starlink, but won’t stop nation state attackers from jamming access to GPS.
There are different kinds of jamming. It's my understanding that interference jamming is fairly uncommon nowadays. If it's not perfect, it mainly gets you a reduction in SNR and GPS is already way down below the noise floor, so receivers are inherently designed to mitigate the effects even if they aren't explicitly designed for anti-jamming. Even when they work, spectral filters exist. Broadband and spoofing are what Russia typically use. The defenses against those benefit from lots of processing power and large, beamforming arrays.
Jamming isn't destructive interference, which would require knowledge of the exact signal being sent as well as the exact location of the transmitter and receiver. Jamming is overwhelming the receiver with a stronger signal in the same frequency band so the weaker one cannot be received. Think someone screaming over someone else whispering.
Phased array beamforming absolutely helps make jamming more difficult. Jamming is all about reducing the signal to noise ratio in the channel until it is unusable. Directional antennas (of which phased arrays are electronically steerable versions) have more gain in the direction of the desired signal and less towards unwanted signals located in another direction.
