He is trying to take out a loan here as well. One of the letters included in the story is actually a margin loan commitment from morgan stanley. But that was for only 12.5 billion. This was all he could get against his shares.
It does seem low in comparison with the value of his shares, but banks are well aware of the self reinforcing negative spiral a margin call can have on a share price and will not give out massive margin loans.
My point is that even after he got the maximum loan he could get, and then got the maximum margin loan against his shares, he is still 21 billion short.
The loans have different risk characteristics than his equity stake, he balances risk by deciding how much of his own money to put in vs. how much borrowed money to put in. (There is no doubt a ceiling — how much of his own money he is able to put in — but there's no reason to assume he wouldn't invest less than that ceiling.)
If you have enough cash to buy a house, you might sensibly still take out a mortgage, and you'll need to decide how much cash to put down as a deposit vs. how much to borrow. (The bank will likely set a lower limit for the amount of cash, but you might choose to put down more.) Someone could ask you "why did you choose that number", and you'd give a similar answer.
You don't know that there is a 21 billion shortfall. The source of funds for his intended equity stake is unknown to us at this point. It could be his imagination (especially given his history with the phrase "funding secured"!), or it could be something we are not aware of.
It does seem low in comparison with the value of his shares, but banks are well aware of the self reinforcing negative spiral a margin call can have on a share price and will not give out massive margin loans.
My point is that even after he got the maximum loan he could get, and then got the maximum margin loan against his shares, he is still 21 billion short.