Writing about satisfaction, St. Thomas says that Baptism confers justification ex opere operato – that is, "in virtue of the deed." But it is not man's deed; rather, it is God's.
Baptism confers justification in virtue of the deed [ex opere operato] which is not man's deed but God's... [ST Supp., Q14, A3, ad 2]
In what way, then, does this order of things suggest that we believe that we save ourselves by our works? It doesn't. Hence those who say otherwise would seem to be misinformed.