Personally, I don't think clarification is needed for the help text. It is clear from context and adds more confusion than it clears up confusion.
Microsoft's explanation of
Keyboard shortcuts in Microsoft Edge mentions:
Ctrl + Tab
Switch to the next tab
Ctrl + Shift + Tab
Switch to the previous tab
It doesn't mention "open" here. But it does mention "open" when referring to the whole group of tabs:
Ctrl + Shift + D
Save all open tabs as favorites in a new folder
Opera does the same
here:
Cycle forward through open tabs
Ctrl+Tab ⇥
Cycle back through open tabs
Shift ⇧+Ctrl+Tab ⇥
It does make more sense in these cases.
Google uses "open" in the explanation of the
Chrome keyboard shortcuts where it does not seem necessary:
Jump to the next open tab
Ctrl + Tab or Ctrl + PgDn
Jump to the previous open tab
Ctrl + Shift + Tab or Ctrl + PgUp
Both ways seem acceptable. However, if "open" were to be used in the descriptions of TC's cm_SwitchToNextTab, I would use "last visited open tab" (meaning among the open tabs we switch to the one most recently visited) rather than "last open visited tab".