What worries me is he didn't say anything about OEM.
All he did say was Windows 10 embeds its licence into the Motherboard, it is then tied to that Motherboard, They consider the Motherboard to be the computer, if you change the Motherboard its no longer the computer the licence is tied to, its forfeit.
I told him the Motherboard is only one component of a computer, he said its the main component and recognised as 'the' computer.
So i don't trust that even a Retail Copy of Windows will be Motherboard transferable in 10 format. the licence with that OS goes with the old Motherboard, its SN is what MS have on record as what the OS was activated on, Change the Board, connect to their servers for activation and what they get back is a different SN, = different computer.