Businessperson: Because the parking area directly in front of the building was closed for maintenance today, I was late to my meeting.
Conclusion
If the maintenance had been done on a different day, I would have gotten to the meeting on time.
Evidence
Because of the maintenance, I wasn't allowed to park in the parking area directly in front of the building. After finding out that I couldn't park there, it took me 15 minutes to find a spot, which made me a few minutes late.
Evaluate
The author's conclusion is about a counterfactual ... if they hadn't been doing maintenance that day
So to argue with him, we have to enter that alternative universe and think, how could we argue that in a world where they weren't doing maintenance in front of the building, he still would have been late?
We know that he got to the lot around 15 minutes before his meeting (because a 15 min delay in finding a spot made him only a few minutes late).
We'd pretty much be trying to think of storylines where arriving to the parking area 15 mins before the meeting isn't long enough.
Maybe he still would have needed to cruise around 15 mins to find a spot? Maybe he would get caught in conversation with people and walk into the building? Maybe he never really has access to the front lot (either he's not important enough at the company to have a premier spot or he gets there late enough that the front lot is usually already full).
Goal
It's hard to know what to predict here. We just know that the author is Assuming a Difference.
He's either assuming that if the maintenance hadn't been going on, he would have been able to find a spot in the front lot, or that it would have taken less than 15 minutes to drive around and find a spot.