This is just to scroll the tab bar when too many tabs are open to see them all.
Switching tabs via mousewheel on its own seems like something that would be too easy to do accidentally, as well as not being standard in things like Chrome at least.
On my last two mice, I have configured them so that holding the mouse's "shift" button (different name on different mice, but the same concept) makes the mousewheel send Ctrl+Page Up and Ctrl+Page Down for switching tabs in pretty much all software, including Opus and Chrome. (And "shift"+RMB does Ctrl+F4 to close the current tab.)