This is at the core of Superman III. Richard Priors' character notices his paycheck gets rounded down, he writes an app to harvest up the rounded down tenths of pennies...
Which is one reason I suspect it might not be true: not because Block is necessarily honest, but because everyone and their dog has heard of this scam and it's the first thing people would think of.
The twitter thread they published appears to contain supporting evidence. Or perhaps there was enough smoke to justify the short position which they covered at market open for a nice return.
Company I write software for had a small subset of customers begin complaining about fractional cents when New Jersey implemented a sales tax of 6.625%. To be fair, company gains and loses on the half cents this caused were in equal measure. But I was commissioned to turn literally everything from UI to database to receipts and invoices and annual reports into thousandths instead of hundreds.
This is a fuckin dog boarding company I work for, not a bank. Because no, you can't screw your customers a half penny at a time... it's on you to fix the accounting and make it right.
