The SAR ADC is also a feedback circuit and thus it tries to make a virtual short between the two inputs of the comparator. It doesn't really matter that in the diagram you've connected the inverting input to ground. You may as well consider it some fixed voltage, say VDD/2 or Vrefm if your comparator is supplied by by VDD and 0. Remember that not only is the comparator input connected to that voltage, but also is the upper side of the CapDAC connected to it during sampling, so you sample the input with respect to that voltage. Of course, you will need to connect the switches to that Vrefm or VDD/2 as well, so everything is consistent.