The last version of my script with the sort by length optimisation is broken. It starts each search into the big string in slightly the wrong places. Hence why it doesn't find as many words as it should. It could be fixed, but it will never be as quick as your approach.

More intriguing is if your list contains 'indistinguishablenessess', & 'indistinguishableness', why doesn't yours output the latter? I didn't expend any time on it, but I couldn't see how it could miss.