Maybe you displayed the ability to not go into debt unnecessarily, which could be also a signal of financial ability.
Getting into debt just to demonstrate you can pay it off is a bit like having to catch a virus so that the doctor can tell you you're healthy once you're done with it.
But it's true that in the era of negative interest rates, only silly people don't owe money.
I guess using the analogy — your doctor won't tell you "you've a great immune system" if you've never caught a bug at all. Maybe you've a really weak immune system, they just don't know.
Not going into debt unnecessarily is a great thing, but it's not a data point in your favour, it's the absence of a data point.
A credit rating basically isn't considered for a mortgage in the UK. They will check a report, and maybe request you pay outstanding debts (I was asked to clear my CC balance before completion both times), but unless you have a fraud marker or are bankrupt it won't affect you. Mortgages are secured on the asset, so it's far less risky than a personal loan of 5x less - the bank can seize the asset and force the sale of the home (and that difference in risk is reflected in the difference in borrowing rates).
My experience is they do indeed dig into your credit report and will approve/deny based on it. Not sure where your information to the contrary comes from?
Most UK CC T&Cs state explicitly "If you have a home, we will go after it if you default." Personal loans have similar recourse.
Neither are truly unsecured. It just takes longer than an outright repossession because it takes two or three court claims instead of just one.
The debt collection industry relies on this, and regularly forces sales of property to pay back CC and other debts.
So CCs are disastrous for people who can't pay their debts. Not only are the interest rates extortionate, but for lenders they're almost as safe as a secured loan. And because of the high rates they can be considerably more profitable.
But it's true that in the era of negative interest rates, only silly people don't owe money.