Throttling by IP address may have worked 10 years ago, unfortunately it's not an effective measure anymore.
Modern cred stuffing countermeasures include a wide variety of exotic fingerprinting, behavioral analysis, and other de-anonymization tech - not because anyone wants to destroy user privacy, but because the threat is that significant and has evolved so much in the past few years.
To be entirely honest, I'm kinda surprised Google didn't require javascript enabled to log in already.