They're saying that it was an unauthorized third party (so either someone put it up themselves, or the council fucked up, most likely the former imho).
The force claimed if a motorist was to have traveled through the section signed as 40mph at the maximum permitted speed of 40mph, then sped up to 50mph after seeing the now-removed fake sign, their average speed on the section would not have resulted in them being issued with a speeding ticket.
That seems like a pretty important part of it too. They're saying that even if you followed all the signs posted, your average speed would not have netted you a ticket.