In the context of corporate finance, the tax benefits of debt refers to the fact that from a tax perspective it is cheaper for firms and investors to finance with debt than with equity.
For example, a firm that earns $100 in profits in the United States would have to pay around $30 in taxes. If it then distributes these profits to its owners as dividends, then the owners in turn pay taxes on this income, say $20 on the $70 of dividends. The $100 of profits turned into $50 of investor income.
If, instead the firm finances with debt, then, assuming the firm owes $100 of interest to investors, its profits are now 0. Investors now pay taxes on their interest income, say $30. This implies for $100 of profits before taxes, investors got $70.
So yes, Jim and INEOS have more than enough to finance the takeover, but to finance it via debt actually benefits INEOS. It has no material effect to United because the debt is with INEOS, so the club is effectively debt free.