Nothing to do with that. If a shareholder lends as an individual it will show in the company's accounts as a shareholder loan. Money moving between group companies happened in very group I ever audited or worked for. It's absolutely routine, as is the occasional writing off of balances. It's simply very visible.
The tax advantages have nothing to do with taxation of trading profits. In any event, Sunderland won't be paying any tax for a very long time, because it has around £200m in unutilised tax losses to offset against future profits. The advantage comes in the treatment of any capital gains made on the sale of any investments made by that company. If they make a large gain on the sale of Madrox, that profit goes, proportionately, into their indvidual companies which they set up to hold the shares. This is where the saving to them as individuals comes in; if they are the sole shareholder of that business (tick), and have owned it for at least two years (tick, as of a couple of months ago), then any gain arising when the company is sold or liquidated is taxable at 10%, rather than 20%. That's why the ownership will pass through companies until you reach the top ones owned by the investors as individuals.