First off,I think that the people of Metru-Nui have legends of the Toa on Mata-Nui,because someone told them about them.But who could without having a connection to the Mata-Nuians?Well look,Mata-Nui is the spirit that created the island,but Metru-Nui could be the one who made...Metru-Nui,but thats not the connection yet,if the 2 have the power to create Matoran and islands,they are both like supreme dudes,now looks at the Nui and the first name of Makuta,Mata-Nui and Metru-Nui.They each start with an M.And Makuta and Mata-Nui are brothers,Makuta has powers like Mata-Nui,Metru-Nui could be Makuta and Mata-Nuis other brother,before Mata-Nui was put to sleep,Mata-Nui told Metru-Nui of the Toa,and he told the Matoran of his place of them.And now they have legends of the Toa.
Yeah,I know,no one mentioned them having legends of the Toa.Its a theory that I just came up with from nowere.