Absolutely and totally untrue. Period.
XML + XSLT. Done. There is the software that does this. It's not the only one, or even the one I would pick, but it is an effective counter example. You maintain one source file and generate any number of formats you want. Each translation is written once.
It sounds like you haven't used it for this purpose.
Think about how you would use XML and XSLT (and presumably XSL:FO for printed output) to produce chapter 2 of the 4e PHB. You need to reproduce the printed layout exactly, including the text wrap around images, high-quality typesetting, etc. You also need to produce an HTML or other version which looks good on the Kindle (a b/w device) with the tables and appropriate graphical elements.
Getting body text to look adequate on small devices will be easy. You've got low standards, and the device handles the formatting. Getting the 34-line Character Advancement table will be hard, but doable if you can just target the Kindle. If you need it to work on the Kindle, the iPhone, Windows Mobile, etc, it'll be substantially more difficult. Getting images to work equally well will also be difficult, since the readers all have different size screens.
But where XSLT and XSL:FO really fall down are in the visual design area. XSLT does not handle printed output. XSL:FO does. There's only one XSL:FO processor I know of which can produce the typographic quality of InDesign, and it's a rather cumbersome tool built on LaTeX. And even it can merely give you good-looking line-breaks, pagination, kerning, and some microtypographical stuff. But since it only handles XSL:FO, there's a lot it can't handle (take a look at
XSL Formatting Objects - Wikipedia, the free encyclopedia for a list).
Since InDesign can import XML directly, it might be an adequate workflow to import it, then hand-tune for print and PDF. However, if the source document changes, you will still need hand-tweaks to make sure it looks good. There are some subtle typographic choices made in the PHB, as an example, which could not be determined automatically. For instance, look at page 110 where the arrow was allowed reach into the first column because there was no text there. If the Miss section of the previous power was removed, that choice would need to be revisited, because automatically reflowing the Evade the Blow text would look bad.
Automatic translation into multiple formats only works today if you have low standards for how it looks in each format. If you believe differently, try it.