You need an ilvl 90+ amulet to get +2 class skills.
The 93 is for gambling amulets to craft caster amulets. At clvl 93+, gambling blue amulets and then crafting caster amulets with them guarantees all resulting caster amulets will end up ilvl 90+ (at level 93 you gamble amulets from 88-97, and then when crafting at level 93, even the 88 gambles end up resulting in an ilvl 90 caster amulet after applying the crafting ilvl formula).
For straight gambling instead of crafting, you don't need to be 93. You just need to be high enough level to gamble ilvl 90+. Gamble ilvl follows a uniform distribution of clvl -5 to +4. So you'll first start seeing ilvl 90 gambles at level 86, but it will only be 1 out of 10 gambles on average that are ilvl 90. Each level you gain will increase your chances of gambling ilvl 90+ amulets (10% per level), until level 95 where you will get entirely ilvl 90+ gambles.
So at level 84, you cannot gamble ilvl 90+. At level 86, 10% of gambles will be ilvl 90. At level 87, 20% will be ilvl 90+. And so on up to 100% ilvl 90+ at level 95.