The problem with this theory is that giants quite clearly know Luffy has eaten the Gomu Gomu/Nika fruit, many of them saw him transformed on Egghead and they even told him he looked like the Sun God.
Of course. If the Gomu Gomu was the legendary fruit, any giant would know Luffy had eaten it then, and not Loki.
Which Isn't the case at all.
The theory works only if the legendary fruit is another fruit and someone different ate It, and news of this mysterious person too never reached Erbaf.
Which It doesn't make much sense since there would really be no point, we were told Loki ate It and the giant should really have seen Loki's powers.