I believe other banks have had secretly bought their own stash of bitcoin in the previous years. Of course, they won't admit it at first because it is like against their financial system in some way. But whoever or whichever will join this market is up to them. As everyone is free to join in this market, without judgment or anything, it is now up to you how you will strategize or take advantage this market.
Stop coming up with ridiculous theories like governments, central banks have been secretly buying bitcoin for a long time and they are not announcing it. That cannot happen at the corporate, corporate level, let alone the national level.
Because fundamentally, corporations must publicly report their financial statements to shareholders, investors, and regulatory agencies like the SEC. Meanwhile, for the government and the central bank, there needs to be bipartisan consensus. They cannot quietly use taxes and national budgets to make indiscriminate purchases without the approval of Congress.
Even agreeing to extend spending to reopen the government took more than 43 days. Or passing a "Bitcoin Act" also takes a long time and no country has even achieved that yet. How can they buy bitcoins secretly? Who has the authority to do this when power is divided under the separation of powers rather than concentrated in one person or one body?