You’re right, it’s very likely he wasn’t intending to use a slur. But it seems to me like a lot of the reporting is “he didn’t mean to disrespect people!” when that’s not the case. The pope’s intentions were absolutely to disrespect people, just by his actions, and not by that specific word. the specific phrasing he used to do so doesn’t really matter.
What you are describing is called “language”
“You” wasn’t always allowed to be singular. Colour vs color. Doughnut can be donut. Etc. Languages evolve over time, and “drive-thru” is in plenty of dictionaries.