That the special Marine narrative was always garbage headcanon pushed by the Yankers.
The Hero title that in canon may be fraudulent and undeserved in itself is the main reason Garp reached a bounty equal to an Admiral.
Couple it with the fact that the weakest C3 Admiral took away what was perceived as Garp's greatest strength, his physical stats, and you have what we've been preaching for years that Prime Garp was "just" a C3 level Top Tier.