Not all parishes have a parish hall. We didn’t when I was growing up. It looks the this parish did and I am not sure why the visiting priest didn’t offer that room.
I also don’t know where the the parish is located. Deep South Texas, Alabama or other southern states would know not to schedule activities in July or August. Northern states, maybe not so much. Go far enough north and you will find most houses don’t have air conditioning. People wouldn’t think about the heat when planning activities. Upstate New York had temps in the 90’s in July, I don’t know what the heat index was.
And I defend the practice because growing up, we showed movies, had concerts and everyone celebrated their services, in the same room. Sometimes there isn’t enough room for different buildings for everyone.
All I can picture is a group of people, outside, sweating, dreaming of a place where they wouldn’t be so hot. Depending on where this parish is, it could have felt like it was 105 degrees. They were just looking for a place to be, inside.