Translation:Marcus arrives on time to the temple.
Yes, ad appears on the verb (advenit) and is used to start a prepositional phrase (ad + templum ).
The verb advenit is still intransitive; it doesn't control a direct object. (Compare how in English you don't "arrive the temple," but rather "arrive AT the temple.")
It's common in Latin to have the same preposition (or a form of it) used as a prefix on the verb: cum amīcīs colloquī , to speak with friends.