The news, genuine or otherwise, comes from Ian McGarry, rather than Duncan Castles, as is clear if you listen to the podcast. 26 mins 8 secs is the starting point.
While both of them are typically critical of United, in this particular podcast, Castles gives a relatively balanced assessment of United's approach towards their recent Football Director and Technical Director appointments, then insight on Eric Bailly being offered a new deal and Maguire's lack of pace preventing United from adopting a different style of defending. Castles also makes clear that he doesn't rate Solskjaer in the slightest and considers the decision to spend £80m on Maguire to be a big mistake.
Personally, I am deeply sceptical of the idea that United are going to spend heavily in the summer on a CB, especially given the number of clean sheets we have had against the top teams. We have a huge number of scouts, so of course they will be doing their job, looking at possible options, should we decide that we have to buy.