The Swamp Lemon leaves were intermediate between the pure trifoliate and the citrange leaves. Not proof, but suggests to me that it isn't pure trifoliate.
That thought entered my mind when I looked at the picture as well.
It could just be a seedling of citrange, or possibly there may have been some cross-pollination with the trifoliate population growing out in the wild, and then the plant that had fruits without a bitter taste were positively selected for by the birds, and spread faster than normal trifoliate would.
I'm thinking, for example, that maybe there was someone who planted a citrange tree outside, or maybe it came from a rootstock that had overgrown its scion and the owner did not realize it, probably a potted orange tree brought inside during the winter, and then a bee carried this citrange pollen to one of the surrounding trifoliate plants in the surrounding vicinity. Or maybe the pollen came from a potted Meyer lemon, and the trifoliate x Meyer lemon offspring managed to survive and grow to eventually blossom, and then to pollinate a trifoliate tree growing in the wild. Then one of the seeds from that fruit grew, and the animals spreading the seeds much preferred the taste of this new trifoliate variety, so there was genetic introgression into the local wild trifoliate population.