Sadly, this is not true. There is actually not a real ‘Universal’ wax. There’s a whole bunch of wax techs, that would probably be out of a job if there was such a thing.
The real truth is that it is not possible, with our current products, to cover such a huge temperature range with a single product. The universal wax products are actually, for the most part, a fairly unrefined hydrocarbon (paraffin) wax, with a few additional components. The effective temperature range where performance is delivered, is a fairly narrow band within the wider quoted temperature range. In practice a wax that quotes -20 to +15 will show a measurable performance increase to an unwaxed ski over the entire range, whilst giving perhaps a better performance increase over the range -10 to 0.
If we are interested in performance gains, a wax suited to the snow conditions will always give a better end result. However, we can also state categorically that any wax is better than no wax!
As per usual, there are no magic bullets and no unicorns either.