Thanks for the suggestion. I tried this in Firefox by clearing all the website data and then opening a private window. I then logged into my account and tried to make the points purchase. No luck, unfortunately. I still just get the 20% off offer for using an IHG credit card.

It looks like points.com checks to see if you are an IHG credit card holder. If so, it overrides the 100% bonus offer and presents the 20% off credit card offer instead. There is a link that asks if you want to use a non-IHG credit card. Clicking that takes you to the default points purchase page with no bonus at all.