The text distinguishes a thing's capacity (potentiality) to produce an effect from actually producing that effect, using the capacity for intoxication as an example.
Compare:
“This seed can develop into a tree” — a potentiality claim.
“This seed is actually a tree now” — an actuality claim.
The first can be true while the second is false, without their being exact negations.
For contradiction, compare potential with potential, or actual with actual. The contradictory of the capacity claim is that this seed lacks that capacity.