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Strong Style was a marketing term created by Antonio Inoki to make his brand of wrestling more legitimate, not all that different than the term sports entertainment, just in the opposite direction.
It basically meant that the way Inoki and others in New Japan wrestle is the the strongest style of fighting, and it could beat boxers, karateka, judoka or any other martial arts practitioners. People talk about 2000s Inokism where he sent his stars off to lose MMA fights they had no chance in and then brought in MMA guys to have fake matches where they still beat his guys half the time. But New Japan had always used guys with legit combat sports backgrounds, their just wasn't mainstream competitions for him to send Riki Choshu to get murdered.
Now Strong Style just means wrestling in New Japan. The modern heavyweight style is far more influenced by AJPW/NOAH and New Japan's junior division that it is by Inoki, or even the Musketeers.