You’re mistaking a probability rule for a logic rule.
A and B can not happen at the same time
But that does not mean A x B = 0
A = probability it rains tomorrow
B = probability it does not rain tomorrow
Since one and only one of these must happen, A + B = 1
And we are being asked: what is the maximum value of A × B?
This is not about mutually exclusive events both happening.
Instead, we’re looking for the maximum possible value of A x B, under the constraint A + B = 1.