Earlier quoted context omitted.
The two companies would likely have more total value separate than together. The stock market doesn't price conglomerates correctly.
Citation needed. When companies are split, sometimes they gain value and sometimes they lose it. There's no hard and fast rule, and I'm not aware of evidence of systematic downwards mispricing of conglomerates.
It is not mispricing, but it is a discount. Big companies, even very healthy ones, often split to satisfy their investors based on this phenomenon.
It hasn't really played much in tech though which might be why you haven't heard of it, but is very very well known in industrial companies.