With a cathode ray tube, the ZX Spectrum has a progressive display. With a modern LCD display and careful CPU timing, it is possible to create an interlaced display. You can read more about it here: https://www.facebook.com/groups/1066251293516270/. But ULAplus only specifies how the palette is defined, not how the video display is generated. The titles in this compilation don't use the LCD-interlaced mode.