Seeing this again in gif form, I'm struck by how much bigger White's hand is. She must be immense in size.
She's probably a fusion
but she's clearly not Blue and Yellow fused, she was shown alongside them when they unleashed the corruption attack, remember they were in normal colors which signifies that it's something that Garnet saw rather than something she heard about.
Their retellings of things can't really be trusted by this point, though.
Considering the number of secrets that were kept, things taken as fact that were straight up lies, gem culture's views on fusion, and White apparently not being visibly active for quite some time, even during the trial of someone who supposedly shattered a diamond, and the color white being the absorption of all colors on the spectrum, she's probably a fusion.