*Usually* to be valid a will has to be witnessed and notarized in proper form. However, there is such a thing as a “holograph will” - if it’s entirely in the handwriting of the testator, even if it’s not witnessed, it can be admitted to probate. But the news article doesn’t really say. Still, it sounds like it may have been a holograph will.
There are pre-printed Wills and all that stuff available online. Very official.
There’s one problem. The only way to find out if they are any good, is to die, and then see if there’s an issue, by which time it’s too late.
Another common problem is people keep their Will in a safe deposit box. But when someone dies, the bank seals the box. Only the Executor or Executrix can gain access. Power of Attorney won’t cut it. And the Will is what specifies Executor of the Estate. Oops.