MyCE claims these screenshots don't show off the new Modern UI titlebars that were previously rumored by Neowin because that is a feature from a different FBL. The site explains once a feature is stable enough it gets merged into the Winmain branch and then back into the different FBL branches.
The first screenshot shows the new Start Menu while the second shot illustrates a windowed Store app.
You may notice that this build doesn’t include the Modern UI titlebars previously rumoured by Neowin, that is because different features are built in different FBLs, once a feature is stable enough it gets merged into the Winmain branch and then back into the different FBL branches. Uip_dev is not the only lab where the stuff like windowed apps is being worked on, the new titlebars are quite jerky in some of the current builds, that also applies to current uip_dev builds which for now resort to the regular titlebar until the proposed modern titlebar functionality gets a bit more fine tuned.