Convenient for the author to discover the link to Le Roux. Then talk about it on podcasts while promoting his book, and have articles written about him.
All Shkreli does is promote himself to get eyes on whatever he's doing next. You can't ignore the incentives. It's difficult to prove who Satoshi is or is not. Craig Wright made a succesful career out of it.
Also, Shkreli has been critical of Bitcoin. Picking Le Roux, a criminal, as Satoshi helps to paint Bitcoin in a bad light.
He explains it during this stream here:
https://www.youtube.com/watch?v=gjE4dEX2z9E
You can just watch the first 10-ish minutes to get the gist.
He gets the same push back he's getting here. Still confidently says "Paul Le Roux is Satoshi."
He explains it during this stream here:
https://www.youtube.com/watch?v=gjE4dEX2z9E
Someone else suggested he could have bought the signature. It wouldn't surprise me.