No, I don't think St Severus says that.
It is one thing to say that the humanity is hypostatic, as he does, and another to say that it is a hypostasis.
I think that for us hypostasis means the inner identity, where prosopon might mean external relational identity. For us, Christ is not only God the Word in an external relational sense (as Theodore of Mopsuestia would have agreed). But he is God the Word in an interior reality of identity.
The humanity is hypostatic, because it is a real instance of humanity with an interior identity, but that interior identity is God the Word who is incarnate in his own humanity. St Severus uses the concept of en-hypostasis before Leontius (even if there is debate at the present time about what Leontius actually means). We can say that the humanity of Christ finds its interior identity only in union with the Word of God because the Word of God is the interior identity of the humanity.
We can say that 'this man' is 'God the Word incarnate', not because the humanity is in an external relationship of grace, or favour, or will with God the Word, as Theodore proposed, but because the humanity is in a relationship of interior identity or hypostatic union with God the Word.