The "financial sphere" and the "real sphere" are pretty closely related: the problem governments have is not so much borrowing constraints as how to effectively deploy the unused 8.1% of the labour resource without driving up prices for related economic resources that are pretty close to full capacity. (rough analogy: not using CPU to it's fullest capacity doesn't necessarily mean you can make a program run any faster if you're I/O constrained)

"Modern Monetary Theory" economists have a lot of interesting ideas (I like economic iconoclasm myself and have formally studied the more conventional stuff) but are a bad starting point for learning about economics not least because they consistently (and deliberately in the case of the academics) misrepresent the positions of the academic economic mainstream.