No, players can be extended.
What it excludes is attempts to get around the IFA maximums. At least that is what every website says.
I've read in a few articles that exceptions are capable of being made in a player like Ohani's case. Even that it might be likely because the rule isn't intended to stop what he is, a player universally touted as an MLB-caliber, and quite possibly one of the world's best right now.
From what I can tell it's intended to stop big-spending teams from securing a young prospect's rights by offering them what other teams around the league can't (more money).
Yoan Moncada, as an example, still spent 2 years in the minors before making his debut. So he was not deemed MLB ready when signed, and none of the 16-year-old Latin players that are signed would be either.
These are the types of signings they are trying to curb with the rule, not someone like Ohani.
Scouting Report: Power: 30, Hitting: 50, Arm: 60, Defense: 45, Speed: 45. "Line drive swing and shows good contact and on-base abilities. Double's power at his peak. Strong arm from 2B or the OF, stiff hands. Not a fast runner, but above average instincts on the bases. Skinny body doesn't look the part, but will sneak up on you. ACL surgery sapped much of his athleticism." (Probably)