There’s actually a really good reason for that. The body doesn’t have a good way to get rid of excess iron except by bleeding, so it’s fairly easy for someone without a period to get iron poisoning from vitamins with iron in them. Women’s vitamins assume the person taking them loses a significant quantity of blood every month. Not only should men not take them, women whose birth control eliminates their period completely shouldn’t take them either.
Researchers have done sonograms of women before and after arousal, with them having used the restroom immediately at the start of the process to control for initial urine levels. The bladder of a squirter fills rapidly with liquid prior to squirting, significantly faster than it does in similar non-arousal conditions. When you pee, you never void all of the urine in the bladder, so there is definitely pee in squirt, but the urea content was significantly lower than in the pre-arousal urine. IIRC, the researchers determined that the added liquid is mostly just water.
So, squirt is pee, but if she’s gone to the bathroom recently, it’s very diluted pee.