The 35% rumor comes from the worldwide employees. Do the the math; Seagate & WD have roughly 40% ish market share (WD seems to be always 2% higher).
Worldwide employees for Seagate = 50K and Western Digital = 75K.
There is a lot of redundancy now post HGST merger. Both companies were vertically integrated and had multiple campuses doing the same thing. I know of a few groups that have 4 campuses/factories manufacturing/producing the same part of the process, be it in heads or media division. Probably a lot more I'm not aware about.