Because Oda has tried to connect (retcon) everything to haki .
Even in the vivre cards if I am not mistaken both Shirahoshi and Momonosuke were given CoO. So pretty much VoaT is CoO and whatever Shirahoshi has as well.
Zoro's Alabasta scene turned into haki and Oda later in the Jinbe vs WW has Jinbe say this
You either take this at face value considering all the things Oda has done recently or simply Jinbe comparing them since both CoA and Tekkai do the same thing;protecting the body.
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Considering Lucci's rokuลgun is pretty much ID I incline to believe the first.