Just because you get elected to be an MP doesn't mean you're bright, well-informed, have an inquiring or open mind. People with very ingrained and incorrect viewpoints get elected as MPs and they can be as stupid, poorly informed and ignorant as the rest of the population. Therefore, it doesn't surprise me that some continue to believe this nonsense.
What I consider unforgivable, however, is that the Lib Dems have STILL not put out a statement of their support for the Cass report. An entire, mainstream party with 72 MPs continue to support the medicalisation of so-called 'gender dysphoria' in children in the face of overwhelming evidence that it does more harm than good (I know the Greens also support it, but they've always been a load of fringe crackpots).