It's a lie.

Tory shadow justice secretary Nick Timothy said on Friday that he had been aware of such an incident when he was chief of staff to Prime Minister Theresa May. “When I was in No 10, somebody failed developed vetting before their proposed appointment to a sensitive post. We were told immediately, and were advised — correctly — that this person could not be appointed. Who gave that advice? Sue Gray — later Starmer’s chief of staff,” Timothy said on X.

Allies of Olly Robbins defend handling of Mandelson vetting - https://www.ft.com/content/58c01bc9-12bf-440c-9a91-01c130a6a9c1?shareType=nongift via @FT