That industry relies very heavily on referral fees to this day. Part of that is matching consumer with broker, and part is brand.
Today it's places like comparethemarket, back then it was Prudential.
So nothing's changed, the web hasn't made any efficiency gains.
The better solution than what they came up with is to have put in a unique forwarding phone number per affiliate, and charge the affiliates a referral fee per call (or perhaps per unique telephone number).
Whether that tech existed in the late 90s I don't know, but it's been available for over a decade now, and was probably available back then in a more analog form. What's nice is that now you get a webhook telling you about each call, so all the billing is automated.
As for when you put your email address into a form on some comparison site? Mortgage brokers bid on those leads. It used to be in the region of £20-50 per lead, pre-2008, and I know someone charging a comparable rate now, plus a cut of commission for subsequent remortgages.
But the article did say that this was at a point in his career where he might not have been experienced enough to make that call.