I think the swiping back on new tab feature is a very well thought out piece of UX.
If a user taps a link that is set to open in a new window/tab, while the bottom/top (depending on user config) URL bar does animate to show the transition, the user may still expect to be able to navigate back to where they came from (especially in such a case where they haven't deliberately made the decision to open in a new tab).
I'd argue it would be worse UX for the back swipe to not navigate to the previous page in such circumstances than that it does but closes the tab (which is reasonably signalled by the URL bar animation).
If a user taps a link that is set to open in a new window/tab, while the bottom/top (depending on user config) URL bar does animate to show the transition, the user may still expect to be able to navigate back to where they came from (especially in such a case where they haven't deliberately made the decision to open in a new tab).
I'd argue it would be worse UX for the back swipe to not navigate to the previous page in such circumstances than that it does but closes the tab (which is reasonably signalled by the URL bar animation).