Yes. The act of God's intellect is His substance because His essence itself is also His intelligible species: it necessarily follows that His act of understanding must be His essence and His existence (since in God there is no form which is something other than His existence).
The act of understanding is the perfection and act of the one understanding. To understand is not an act passing to anything extrinsic; for it remains in the operator as his own act and perfection.
Existence is the perfection of the one existing. And just as existence follows on the form, so in like manner to understand follows on the intelligible species. But in God, intellect, and the object understood, and the intelligible species, and His act of understanding are entirely one and the same.