It's not that hard.

She rents the house she lives in for more than the cost of her mortgage. She makes income from that there. They don't own a car. Her husband gets a free bus pass from work. No housing or transportation expenses. So what's left? Food?

Her husband works in tech (first Amazon, and now Google!) and saves 100% of his income. And they live off hers. They have like $1.7 million saved from his Amazon stock which should be a nice investment income too.

Their expenses are around $1000 per month.

Not that hard to imagine the math.