Sounds like more hybrid confusion.
And someone not communicating well.
My read is they were clumsily trying to "do the right thing" by insisting on using the recommended weight oil. But as you know the differences between 0W and 5W are slight and will not have a measurable difference except in _extreme_ cold conditions.
Definitely a walkout, but based on their novice communication skills and knowledge of lubricating oil.
The caution of using synthetics applies to switching from mineral based oils when a car has higher miles. With normal wear switching at 100K miles will likely _increase_ oil consumption (out the tailpipe) due to the characteristic of synthetics. Won't do any damage unless you let the car run out of oil. And the mile number is not an absolute.
But being prudently cautious rather than recklessly fearless is always a lower cost route
HTH!