You can tax something without owning it. It is the owning part that bothers me. It implies that the government is going to shovel ridiculous amounts of money into these things and when the bubble finally pops We The People get nothing out of it.
That sounds like a brilliant idea if you're a board member of one of these companies.
Taxing X% ownership means you get X%. You don't pay for it. So if the bubble pops, you still get X% ownership. To your point, it may be smarter to tax the IPO valuation and buy more later.
I think you've misunderstood the proposal. The government levies a tax in the form of shares, not cash. It doesn't pay for the shares.
FTA: "It would create a sovereign wealth fund through a one-time 50 percent tax — not on the profits of OpenAI, Anthropic, xAI and other companies, but paid with something far more valuable than that: the stock."