Purchasing at the time of fitting?

No

Titleist Thursday time slots are free fittings to you. You have the option to buy at the sponsoring facility or any authorized Titleist dealer. So you can walk if you want. If you elect to go to a pro shop offering both Miz and Titleist to compare side by side, there will be a fitting fee that can usually be negotiated with a purchase of at least 5-6 irons.

The other option is to simply find a demo day when both are there to try both. If you would buy a Titleist option if it beat your current set, then you should consider the Titleist Thursday. It is a fitting not just a demo and someone else
more intent to upgrade could be denied a slot. If Titleist can't improve on your current, then you have no obligation to purchase.