Moral evaluation of his action I leave as an exercise for the reader.
Why is not valuing the feature that much an asshole move?
I'm sure you'll figure out a way somehow. It's amazing how far people will bend themselves out of shape over Steve Jobs.
Of course it's reasonable for anyone in business to say "you're trying to bullshit me, so I'm not doing business with you". The OP agrees, end of story.
But - Jobs might have batted away OP's amateur bravado with grace, and still bought his company for any price he wanted (probably even less than it would have cost Apple to build!). Or he could have passed them over to a subordinate be let down.
Instead Jobs personally wanted to be known for cruel honesty, and for ordering a clone of a startup that he'd changed his mind about buying. I genuinely don't know whether he succeeded because or in spite of that kind of conduct.
Jobs would have had an estimate of what it would take to copy the feature - way less than $50m, just not quite as quickly.
It surely wouldn't have made a lot of difference (to Apple) if Jobs had laughed off the bluster, and negotiated back down to pretty much any number they liked, maybe even less than $50m.
But the asshole vibe from Jobs comes from the fact that he clearly didn't value the feature that much. It was more valuable to wait for his own team to build it slower, and send the CEO away with a flea in their ear pour encourager les autres. So I'm sure that story got heard by plenty of other potential Apple acquisitions, and saved Apple a lot of money down the line.