You should read the whole thing. This makes more sense in context.
Really? I just did, and it only confirmed my conclusion. Luther vacillated slightly on his emphasis on predestination (and double predestination). Calvin does not. He is very clear on this. What part of it do you believe mitigates what I already quoted?
For instance, he considers the possibility that damnation is due to human sin and actually rejects that possibility. Here he is considering his opponent's view, and note especially the last sentence:
It is frivolous in our opponents to reply, that God does not altogether reject those whom in levity he tolerates, but remains in suspense with regard to them, if per adventure they may repent; as if Paul were representing God as patiently waiting for the conversion of those whom he describes as fitted for destruction. For Augustine, rightly expounding this passage, says that where power is united to endurance, God does not permit, but rules (August. Cont. Julian., Lib. 5, c. 5). They add also, that it is not without cause the vessels of wrath are said to be fitted for destruction, and that God is said to have prepared the vessels of mercy, because in this way the praise of salvation is claimed for God, whereas the blame of perdition is thrown upon those who of their own accord bring it upon themselves.
Now he continues, assessing the whole view but especially the last sentence, especially the idea that reprobation has some cause beside God's will:
But were I to concede that by the different forms of expression Paul softens the harshness of the former clause, it by no means follows, that he transfers the preparation for destruction to any other cause than the secret counsel of God. This, indeed, is asserted in the preceding context, where God is said to have raised up Pharaoh, and to harden whom he will. Hence it follows, that the hidden counsel of God is the cause of hardening.
Calvin explicitly rejects the idea that the cause of hardening is something other than the secret counsel of God, namely the sin of the individual.