The synfig user interface has already change since the development version you are testing (migration from gtk2 to gtk3 graphical interface toolkit) … tabs are thinner … but still are tabs .
SO actually, you just can’t remove the tabs … it’s “hard coded” … But you can make a feature request like this one : when only one canvas is opened, do not display tabs …
I’ve posted a request by the link you provided (the link is a bit off and leads to 404 error).
I’ll check from time to time the future developing versions of Synfig.