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.
To simplify this, think of POS blocks no different than ordinary Bitcoin txns, POW miners decide which is valid and in what order.
Except that POS blocks contain transactions as well, therefore imply a consensus?