Kata

Improve your explanation as it is not clear why intervalList1 should to give false answer. Test basic is clearly in proper range, room is not occupied in its time interval.

        StartDate = DateTime.Parse("4/6/2019 1:02:32 PM"),
        EndDate = DateTime.Parse("4/6/2019 1:03:32 PM")
        

if we will confront it with list when the room is occupied

        StartDate = DateTime.Parse("3/2/2019 3:04:32 PM"),
        EndDate = DateTime.Parse("3/5/2019 5:06:32 PM")

        StartDate = DateTime.Parse("4/6/2019 1:04:32 PM"),
        EndDate = DateTime.Parse("4/6/2019 1:06:32 PM")

Initial code needs at least using System.Collections.Generic; with the function signature (List<Interval>).

It looks like there are no tests with partially overlapping intervals like

xxxx
  yyyy

It's not specified whether boundaries are inclusive or exclusive and there are no tests with equal boundaries.

Edge cases not enough

Needs Random Tests

Class Interval should be preloaded