whoever said that is not very well informed.
You can make your rotation fairly similar to live by taking only passive talents, and going Breath of Sindragosa for the lvl 100th talent. But if you take Frostscythe, Icy Talons, Hungering Rune Weapon, and Glacial Advance, then your rotation is DRASTICALLY altered from live.
Not to mention that for AoE, your rotation changes no matter what, due to Remorseless Winter.
Live rotation for frost is very spammy. Just burn your runes on obli, then burn your RP on FS, weaving the occasional HB whenever rime procs, and refreshing your diseases whenever they fall off.
In legion, if you go with certain builds, you gotta alternate between Oblis and FS to maintain Icy Talons at 3 stacks for as long as you can, save your KM procs for Frostscythe, use glacial advance on cooldown, Weave HB whenever Rime procs, use horn of winter on cooldown, and if you've taken the Frozen Soul perk on your artifact, also use Remorseless Winter on cooldown.
Does that sound AT ALL like live to you?
Not to mention that gearing and stat prioritization has drastically changed. Frost DKs on live, value haste above all else. In Legion, Crit seems like the go-to stat for frost.