But it is not personal data that would identify you (PII). If someone was able to determine who I was based solely on my browsing activity on Amazon, then they've already obtained my personal information.
PII is not a term that is used in the GDPR. The person you're replying to is correct that your browsing data is likely to count as personal data given that it's linked to an individual.
No, it isn’t PII in the legal sense, it doesn’t allow identifying you directly. Which doesn’t mean that it cannot be tied to your identity. Just one example: if you regularly post to social media what you bought online, this information could be correlated with the Keepa data to find out which profile is likely yours and what else you looked at.
But GDPR doesn’t merely require you to disclosure collection of PII, but rather all data collected. There is a good reason for that.