I don't know where you got the number of bitcoins from, as it's not been released in any filing and Elon has only commented in terms of USD.
You are right.
My numbers are based on speculations.
I will look at that article and try to improve my numbers. I might have missed some important piece of information.
EDIT:
I am estimating the buys at 39,000 USD.
This gives me, in this spreadsheet, a value of roughly 38,462 BTC.
I see in the article you posted the following:
This looks so optimistic: looking at the Cryptoquant graph I see two major outflows in January, that might be linked to TSLA (message reported also in OP):
If I change the buying level to 33,000 USD, the amount grows to 45,455 BTC.
This would put the valuation at 1Q21 right at 2.48 billion.
Mystery solved?
I think it's a lot closer than it was. It's all still speculation until we can get a hard number from a regulatory filing, but until then everyone is trying to piece it together based on information that has been publicly released. It's like trying to assemble a puzzle where you don't even know how many pieces there are or what shape they're supposed to be.