Yes, you need the mounting plate. The billows has a spring clip in the nozzle that holds it in place. This works as is with the vent hole on a webber kettle but you need the mounting plate to reduce the size of the vent hole on a kamado in order for it to attach.
Great, thank you @HatBeard. Your BBQ posts over the last year played a large part in our decision to purchase.
Did you need the billows Kamado adapter plate and damper vent thing too?
https://thermapen.co.uk/billows-accessories/147-kamado-mounting-plates-for-billows.html
https://thermapen.co.uk/billows-accessories/175-billows-damper.html