This is incorrect for several reasons.
1) The scenario you described is only valid for the POS block being currently mined, however this is no different than POW miners attempting to mine blocks and come up with 2 blocks around the same time. The network will eventually go with one fork and the other will be orphaned. In this case, it is the POW miners who decide which POS block is built atop.
In your opinion it isn't a problem to have a double spend produced every other block, forever?
In POW, doing this has a cost; the cost of block production, making this attack quickly become unsustainable. However, by mixing the two consensus models, you allow double spends at no cost at regular intervals (the POS blocks), which brings the entire chain down to the same security model as plain POS.
So i think you are looking at it from the micro view, where the concern is regarding double spending during the POS block. That is a valid concern.
But firstly, it is incorrect to say that the
whole chain becomes as insecure as POS.
Rather, if you think about it, the combination actually makes the chain far more secure than POW alone.
It basically makes it impossible for a 51% miner to take over the chain unless he also happens to have 50% of all BTC.
Regarding the double spending during POS blocks. POS basically works in the following form:
sha256(output hash + timestamp) = HASH
ValidProof(HASH) ~= Coin age * number of coins
So as far as POS is concerned, either you have a valid block or you don't. There is nothing further you can do about it.
Grinding is not possible for the current block.
Except that POS blocks contain transactions as well, therefore imply a consensus?
Well, txns also have implications on the state of UTXO set and affect validity of future txns as well, so it isn't really different in practice.