When Trew took over Notts, he had quite a large portfolio of businesses that he then sold and pumped into Notts (as loans as well as shares). This chap is "Asset" rich, not cash rich hence the liabilities as he has obviously leveraged against assets to get cash.
The £350m stadium isn't his £350m, he's just part of it. I would certainly state that it's unsustainable as his fortune is reliant on a bubble right now, and bubbles burst