The main reason for that recommendation is that The Colour of Magic and The Light Fantastic, though fun books and enjoyable fantasy parodies in themselves, are really not representative of what the later Discworld books were to become. When Terry wrote Equal Rites and Mort, he really set out his manifesto for the series as being about this world, reflected in that world, and that theme is what really carries forward.

So with the caveat that you need to understand those first two books aren't representative of what the series becomes, you could definitely start at the beginning. Just know that across so many books, there's bound to be some unevenness. Some books are just plain fun, others have deeper messages, some are, frankly, just not as good. Early highlights like Guards Guards, Pyramids, Small Gods, Wyrd Sisters are the ones where you'll discover the real breadth of what he does. Later books can be less consistently original, but when you get to books like The Truth, Night Watch and Monstrous Regiment, there are some real gems.

But don't let yourself not read any of them. That would be a real shame.