I confirm [my attendance].
 I confirm [that I will join the meeting].
Preliminary point: A verb is only transitive if it actually has a direct object in the clause that contains it.
There are many verbs that can be trans or intrans, and "confirm" is one of them. In  "confirm" has the noun phrase my attendance" as direct object and hence is transitive. But in  the that clause is complement of "confirm", not direct object, and hence "confirm " is intransitive.
Some (but not all) verbs can take gerund-participials or infinitivals as complement. These are called 'catenative' verbs, and the clauses that contain them are called catenative constructions. But the clausal complements are not objects, but catenative complements.
For example, in I regret locking the door, "regret" is a catenative verb, and the gerund-participial clause "locking the door" is its catenative complement, not direct object. Similarly, in I want to see a doctor, "want" is a catenative verb, and the infinitival clause "to see a doctor" is catenative complement, not direct object.
Note that with only a very few exceptions, clauses do not function as objects.