I have never seen this term used outside of a car exhaust
situation - but I'm sure it could
The most important thing when installing a exhaust system is the air flow. Now, the exhaust system in a car isn't a straight pipe running to the tail pipe, it's bendy all the way through.
Now, imagine you've got a long tube in you hands, and bend it. What happened? The pipe did some funny things, but the most important thing to note is that it didn't maintain the same shape where you bent it. This will restrict the airflow and make your new exhaust system an expensive waste of time.
What being mandrel-bent means is that a special process is used to bend the pipes that doesn't cause any crimping and thus doesn't restrict air-flow at all.