Where have you found anything "official" from the NDP saying "we never took the riding for granted". maybe they did and maybe they didn't. i don't know, i have not read the press release.
Libby Davies said so on Power & Politics with Evan Solomon on Tuesday's show. She was on a panel discussing the by-election results with Scott Brison and Rick Dykstra.
Obviously no party is going to ever admit it took a riding for granted. Why would it be a surprise that this would be the official position? I would expect any party to say the same.