Having looked at their site, I'm as baffled as you are.

I was amazed to see that the UK was not included in the list of eligible countries. Confusingly, see this list of eligible countries on the Immi site - Immi eVisitor list - which list doesn't include the U.S.! However, that's for an eVisitor visa, not an ETA, which apparently is different and is discussed here - Immi ETA The eligibility link on that page then produces this list - ETA Eligibilty - a much more complete list, including the US and the UK. However, that list distinguishes between countries available for an ETA (which can be applied for by the airline etc. on your behalf) and those that you can do online yourself. The UK appears to be one for which you cannot do it yourself online and have to do it through the airline etc. The airlines used to do it for free but I think they probably charge for it now.

As for the required validity of the passport, see this page - Fact Sheet. I suspect the issue may be that the ETA allows you to stay for 3 months (they don't know when you're planning to leave) and if you stay for 3 months, the passport will have less than 6 months left when you do leave. It's confusing and doesn't specifically mention passport validity requirements other than saying the ETA is valid for 12 months or the validity of the passport if it's less than 12 months - it sounds as if you have to have at least 9 months validity on the passport when you apply.