I am not so quick to judge. Too many unknowns, such as How long has she known?, Just how much does she know? How badly was her first response? Has she been at all supportive? Do you think she was just "Going through the motions" hoping "It's a phase"? What steps have you been taking? What external changes have there been? etc.
Let's be honest here, how long has it taken us to look in a mirror and not see a man? Especially if you are primarily presenting as one? Now imagine you are an SO and had the T-Bomb dropped on you. She had how long to get over the shock and try to grasp a situation that have spent a lifetime trying to get a handle on.
So, of course during an argument she may go for the jugular. A few times my wife has. Even before I dropped this on her! Only the weapon changed. During heated emotional outbreaks things, totally unfiltered and unintended things, may be said. What really matters at the end of the day was the intent. Was she hurt and wanted to hurt you back? Seems like she succeeded. It appears you feel she wanted to hurt you vs just a general "lashing out".
When my wife "blurts" I usually feel it was not intended to hurt. Lash out. Really make a point. Get my attention, sure. Worked big time. I take my lumps and don't immediate react to it. I also learned not to let it fester. When things cool down, hopefully before bed, I revisit those words with her in a totally non-confrontational manner.
At the end of the day our wives believed they were marrying "Men". They believed they were married to men for however long it's been. Transition blows that world apart. Not many can see beyond the image to the person inside. TBH - This person inside has changed a lot, for the better. Just ask my wife! Sure, some aspects she isn't thrilled about. The same ones I feel guilty about. This isn't what either of us expected, nor signed on for. It took a few years of working on myself, help from my TG support group, some therapy, and HRT to bring me to the far better and happier place I am today. The journey was scary for me, even scarier for the passenger, my wife. She mostly likes the improvements. It's been a long time since I heard "Who in their right mind wants to be a 50 y/o woman?". I still get the occasional "I did not marry a woman".
Only you know if it was spoken out of malice, or from love. If you aren't sure, perhaps best to ask her, rather then guess?