Thanks for the note on the edit.
Following a rulemaking exercise under sections 48A and 80A of the Communications Act by Ofcom in 2019, Openreach are indeed responsible for all telephone exchange buildings in the country. Since they're responsible for the buildings, it seems reasonable for them to be marked as the operator. Openreach do rent large parts of their buildings to other operators (including the legally separate other bits of BT plc), but also other operators. Ofcom notes that Cowdenbeath Telephone Exchange also has equipment by TalkTalk and Sky hosted in it.