As haggis pointed out WD40 is a water dispersant not a lubricant. I’d be questioning their skills and experience in removing stuck seatposts using this method. Or were they just having a go and the red mist set in when it was obviously stuck fast.
WD40 Penetrant Spray is designed for exactly this purpose and I’ve used it as such in marine environments.
From WD: “Our Specialist Fast Release Penetrant spray specifically targets corroded or rusted components and mechanisms. It’s formula loosens stuck or seized parts quickly and easily.
The penetrant has an extremely low surface tension, meaning it can cut through rust, seams and tightly-bonded threads to easily saturate and lubricate seized fixings. The Smart Straw and 360° valve allows it to be used precisely at all angles, and the capillary action of the formula means it can reach tight and hard to reach spaces.”
Plain vanilla WD40 not so much, but WD40 is just a catch all trade name these days and not all products are equal.