N1 limit,
I don't know who you fly for but my company's FCOM supp procedures is confusing on this point. You get used to poorly written manuals, by the way. The FCOM states 'approximately 0.76/280' for severe turbulence penetration but then shows a table which states that these speeds are for the climb and cruise only. They advise turbulence N1 for the cruise.
In my humble opinion the tables are correct and the prose is wrong. I say that merely because it would be common sense that your turbulence N1 and therefore Mach is effected by many factors, all of which I presume (I haven't checked because I have to cook supper now) are included in the PI section of the QRH. To have a blanket speed to be used in the cruise at all levels and weights seems a little odd to me.
Personally, I have only once experienced severe turbulence and the notion of setting any particular speed or N1 was laughable when you could barely make out the instruments. However, in moderate turbulence I think it is logical (though I appreciate my logic is often flawed) to set a speed that gives you a safe margin between stall and overspeed.
I think your Captain is misguided.