Except I would have made the exact same assumption about "your father". The idea is that our parents, grandparents or whatever are technologically incompetent. It had nothing to do with gender.
It's heartening to see this thread land so solidly on the intersection of sexism and ageism. You're right, assuming older generations are technically incompetent is wrong, mean, and unnecessary--just like assuming the same thing about women. When you combine the two with phrases like "so simple your mom could understand", you end up with a big bucket of nope.
My mom has a STEM degree, and her mom had a bleeding edge Thinkpad when the rest of my family was married to a beige box. My dad is a system administrator, and his dad is a chemical engineer. I have a postgraduate degree in software engineering. To assume any of us are technically incompetent because of our age or gender is insulting, and exclusionary.
Again, the only sexist thus far, is you.