I'm not quite sure, but I believe he still did hand seals to summon the snake while Naruto jumped at him.

Anyway, if those sand jounins were able to summon their 3 snakes, I'm sure Kabuto could summon one. He seems pretty strong.

And if that's still not enough for you, perhaps Kabuto summoned the snake using Oro's power, the way Naruto performed the Henge seal for Gamabunta.