You’ve done all you can as a host. You’ve offered cake and a discounted/free weekend on future stay. Unofficially, just keep going with that mentality. Most guests will understand home ownership, especially in certain areas, come with bug problems and won’t complain about it on review. Do NOT mention reviews to them personally prior to the review posting. If they call in to support, that conversation can/most likely will be twisted into extortion and can negatively impact you as a host.
Official response you’ll get from support: Thanks for proactively being a great host, rest assured we support you and thank you for letting us know about the ant issue. We will make note of it in case your guest calls in to us as well but for now just continue showing the guests it is your priority and you’re on top of the situation. In order to show our support for you, send me a receipt for the pest control company coming out and I’ll reimburse you up to $100 as a thank you and help to minimize this cost to you.