I also have this "tab bar" option running Sierra OS, but was disappointed to discover that it doesn't actually do anything.
From what I can tell by reading other Evernote discussion threads on the "tab" feature, this has been in demand for quite some time, but hasn't materialized in any Evernote updates.
I agree that the ability to open multiple notes as tabs within the same window, would be an incredibly helpful feature and would definitely improve my workflow. As a Masters's student I often use several notes as a time to collect research and take notes for a given assignment. being able to quickly jump between these within the same window would be awesome.