This is the part that I wonder about. Yes, its easy to verify that they own 100k bitcoins. But the thing is that nobody will know how much they say they lent out. How will we know that it doesn't turn into fractional reserve banking where banks can write loans for 9x of their deposits? The only thing to bring them down, as with the banks, would be depositors fleeing, so in this case, people selling the ETFs shares and hence Blackrock having to dump bitcoin. But this probably will not happen, so it would be quite difficult to catch.
I think you have some things mixed up with the fractional banking system. Not quite like that. ETJ's post is more in-line with securities lending
Let's examine the players
Who are the buyers of Blackrock paper bitcoins? The ETF buyers
If Blackrock ETF sold the equivalent of 1M bitcoins, they are bound by GAAP and all kinds of wall street audit requirements, and auditors will go to their custodian Coinbase and verify the 1M bitcoins
Auditors say these 900K bitcoins you lent out, show us the collateral they provided, commercial papers, treasuries, stacks of $100 newly printed bills
Who are the borrowers? Wall Street traders, Sequoia and the likes
What do they do with the bitcoins? They make jewelry stuff, put diamonds, emeralds and wear around their necks...
Just kidding, they use them for trading the markets, risk free arbitrage opportunities on global exchanges, short here, long here, or if they can do some of those contango or backwardation, sell the bitcoins and go long the futures, lock in some APR's via short term futures settlements
So how does it affect Bitcoin's scarcity? probably not much, depends on global liquidity and market supply and demand
The buyers of the bitcoins that the borrowers/traders sold, what do they do with the bitcoins? Are they short term, medium term, or long term bitcoin investors?
When everything is settled in the markets, depends on the supply and demand of bitcoins
But it's not like fractional banking, where if you deposit $1K in the bank, they can lend out $3K creating new money, new $, fiat ponzi at its finest
Only the Bitcoin miners can create new bitcoins and in limited and diminishing amount