It's not his own words ('Impossible') but how different media have worded their spiel.
Here is the ITV's take with an extract from their reporter quoting Marcs own words.
'I’m not saying that this crossword cannot be completed, because of course it can.
But it is nevertheless fiendishly tough and significantly harder than anything I’ve ever compiled before'
http://www.itv.com/news/london/2018-04-18/can-you-crack-the-worlds-hardest-crossword/
And as you can see from the E-mail that I received, Marc was also not happy about the Title. But publicity and promotion of the book was paramount to the publisher.
In my opinion, The Crossword was a challenge. I've read on another forum that it's not that hard. But the poster is asking for answers which kind of defeats the object of what they are trying to say. If you attain 63 out of 64 of the answers, then the last answer could evade you for a long time (Hence the OTT time scale mentioned) But asking for help would mean that one has already failed that crossword. And also without submitting the finished grid, how would one know if they have the correct solution as the answers are not available as yet.
So I can see (a little) where these quotes may have arisen from.
Saying all that. Good luck with the remainder Davemano.
I hope you enjoy it as much as I did.