I also missed the construction updates, so thanks LTNman. I believe the walkway in your second picture will connect the existing rail footbridge to the DART terminal. In the last plans I saw it was always envisaged this connection would be made, even when the new footbridge is connected up. The slope in the walkway is due to the slight height difference between the existing footbridge and the DART terminal upper level.
Also, if people are interested, Google Earth updated an aerial view of the airport recently (from March 2020) - it shows the DART route, second multi-story, new taxiway and de-icing pads and of course the picture is full of parked up planes from the first lockdown.