That just sounds like an attempt to save both of you time (elapsed time, from interview to offer, not time spent) in case your phone interview did do well.
After the phone interview, somebody needs to write it up, a hiring committee needs to look at it, and then they can decide whether to move forward or not. If they give you the challenge after that it's easily another week.
Frankly I am surprised they gave you a full response on Monday after a phone interview Friday, that's actually quite fast given all the steps and people involved.
I joined a nerdy martial arts club (Historic European Martial Arts), to play with swords, get fit, and meet other people who are nerdy but not in tech. Made many new friends that way.
It's a description of reality. We can agree the situation is unfair, and that the big companies show an arrogant approach, but if you want to get hired you have to conform to what is asked and expected.
Fusion using lasers is an off-shoot of H-Bomb development, and advances by John Nuckolls from early laser-based fusion research in the 1960s(!) were fed back into H-Bomb research.
These fields are surprisingly related. For details, see Alex Wellerstein's book "Restricted Data", chapter 7.
I could see this as a virtual office, as some of the comments speculate.
If I had a set of AR glasses that projected what appeared to be an 8K monitor on top of my dining room table, and integrated with my MacBook Pro for input / output, and that had batteries to last a workday, I’d pay $2K/$3K. Even more if it worked well at brightness levels I’d have in my backyard.
Never mind gaming, mobile high-quality virtual office is good enough.