If you look at the armour from the Patiala family that is reputed to have been given by Guru Gobind Singh it is almost identical to the one being auctioned at Sothbys.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/20777-auction-sikh-gurus-body-armour.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=20777

Given the fact that behind this is someone (probably British) who is trying to make money, it is not surprising that the fact it might be Guru Ji's is being played down. I've met Madra and one thing I can say about him is that he is passionate about preserving Sikh heritage, whatever unorthodox view he may have of Sikh history.