Yes, it is true when it comes to Calvin's soteriology.This wouldn't be true even if your theory about the origins of Penal Substitution are correct.
That just isn't how this works. Penal substitution is NOT a Calvinist distinctive! I don't care what else you do believe, if don't believe that God is utterly immutable or that humans are totally depraved then you ARE NOT any sort of Calvinist whatsoever - period.
Again, just because Calvin taught something doesn't make it false nor does it necessarily make it Calvinism.
John Calvin moved the Cross from the classic position of solidarity and from the Roman Catholic position of merit to a judicial philosophy expressed as divine justice. This is strictly Calvinistic.
You don't recognize this (you might if you start reading Christian history on the topic), and I don't hold it against you. Others who hold Calvinistic views may not recognize theirs either. That is what influence does - it often goes unrecognized.
When I say you are Calvinistic in your understanding I mean it as factual (based on Christian history), NOT as an insult. There have been wonderful Calvinists through history, and great men who held a faith if a Calvinistic trajectory (John Wesley comes to mind) as well.
I point out that you hold a view that was developed by John Calvin only for your edification - not that you change your position, but that you may consider how you approach others who hold different Calvinistic views.
We all have standards. Mine is the Bible when it comes to doctrine, but when it comes to Calvinism it is holding the Penal Substitution Theory of Atonement. That is the uniting factor distinctive to John Calvin.