The only plausible situation I can see is that he or one of his employees was purchasing consumables used as part of the repair business using the affiliate accounts. More likely is that at some point in the past 7 years someone Amazon associates with his account made a purchase using an affiliate link and they used that as the excuse.
Are tech illiterate people intentionally using 3rd party software to change their default browser? Genuinely asking because I have always manually set my default on a new device.