About as much as the word of any billionaire. The issue is we’ll only ever have his word, since Twitter is private, and being honest about financials when poor is bad for business.
I’m willing to believe it’s closer to the truth than not, though. For one cutting out ~5,700 employees, or around ~75% of your work force, is a massive cost reduction. According to Payscale[0] the average salary at Twitter was ~$117,000/yr. A quick calculation shows that annually he shaves off $666.9M (which is hilarious). Alongside decommissioning unneeded offices and removing all the completely unnecessary and expensive time-wasters I can imagine he’s at least saved $1B or more annually.
[0] - https://www.payscale.com/research/US/Employer=Twitter/Salary