On the iOS app for both iPad and iPhones, you can't reorder the tabs. You can't drag them to a different order in the standard view on iPads, nor in the Tab View for either device.
I'm currently on version 1.3.7 (15) on iOS 17.3 on an iPhone 15 Pro Max and iPadOS 17.3 on an iPad Pro (12.-inch) (5th generation). But this have never worked to my knowledge so maybe it's a feature request and not a bug?
A user would use this feature to change the order of the tabs by doing a long touch on the tab and dragging it into a new place in the tab order, either in the tab bar on iPads or in tab view in either an iPad or iPhone.
Safari, Firefox, Brave, and Chrome all include this feature on iOS/iPadOS. I think it only works in tab view on iPhones.