I'm not sure about legally speaking, which how things are legally aren't necessarily the same as what people would consider to be. As a commercial building, I don't know if that's considered part of the interior. What I'm finding is for commercial buildings is that if it's covered, it's included:
https://nces.ed.gov/pubs2006/ficm/content.asp?ContentType=Section&chapter=3§ion=2&subsection=1
What I have hard time getting around is that this was neither caught on camera nor was the perp seen again. I'm trying to figure out how that could be done and one way I see doing that is if LE is being hyper-technical on what they're calling the interior of a commercial building, which covered commercial space in TX may be considered part of the structure and hence technically the interior. It makes things much easier logistically for the perp to already be outside.
Now if they are in a room in the inside SW corner, they could go through a window to the outside, but that too would contradict LE that the perp left the way that they came. Other than that, the room the perp happened to be in is the room that controlled the cameras so the perp disabled one or more interior cameras around the time they killed Missy as they would have had to have disabled at least one hallway camera prior to leaving a room in order to not be seen going out the broken door/broken window that were in other parts of the building requiring being in a hallway to get there. Along with this I don't rule it out that the perp did disable one or more interior cameras (and maybe stole the camera controller) with the footage LE got actually being backup that was stored on the cloud.