It's quite simple really. If said billionaire owner decides to spend, for example, £100m on Ronaldo and pays him £200k per week out of his own pocket, then the club is living within its means (even if the club only has an average crowd of one man and his dog). The same is true if the money is borrowed and the billionaire owner pays it back at whatever time frame is agreed upon. It may seem unfair for a few clubs to have a sugar daddies, but that's life and it doesn't break any rules.
Unless ML defaults on any debts (and by all accounts there aren't even any) Saints are living within their means.
It's when an owner (rich or not) borrows money to fund a squad of highly paid mercenaries for a tin-pot club, and otherwise could not afford players on 1/4 of the salaries, and then goes into administration, paying back creditors 20p in the pound, that's when they were living beyond their means. Not only did they steal from creditors, but also from other teams who lived within their means but were denied a place in the PL or progress in the Cup by the
CHEATING BASTARDS