Regardless of the situation in Home Alone, I would not use the informal "slept in" in any situation. If it were planned, I'd say "slept late;" if it were unplanned, I'd say "overslept."
I think it is a question of dialect. In Canadian English "sleep in" is the universal usage for "sleep late" regardless of whether one's tardiness was intentional. I have heard before that this usage is uniquely Canadian and do not know what other dialects prefer.