If they were going to create a reserve, the confiscation of assets such as this would be the only path to do so imo. I dont see they can generate the funds to buy BTC in quantity otherwise.
Definitely not the only way, but keeping any seized bitcoins would surely make it easier than dumping them on the market just to buy them back later on (although if the timing is right, it could actually work out quite nicely).
But anyhow, if the target size of the reserve is said to be 1 million, and if they were to spread purchases over a longer period, say 5 years, with regular buys (DCA) the annual cost would probably be somewhere around $15 billion, which is a lot, but probably manageable, given it's a one-time expense and the holding would be expected to appreciate in value over time.