The teaching of the Buddha is the unchanging truth whether in the present or in any other time. The Buddha revealed this truth 2,500 years ago and it's been the truth ever since. This teaching should not be added to or taken away from. The Buddha said, "What the Tathagata has laid down should not be discarded, what has not been laid down by the Tathagata should not be added on to the teachings". He 'sealed off' the Teachings. Why did the Buddha seal them off? Because these Teachings are the words of one who has no defilements. No matter how the world may change these Teachings are unaffected, they don't change with it.
I searched briefly for this online and couldn't find it (paraphrased perhaps?) -- I'm curious which sutta(s) Ajahn Chah is referencing here, any help?