Although you are correct, this destroys the engine.
A good, efficient fusion engine just needs to point the exhaust end towards the enemy and the hyper-accelerated particles will punch a hole through the target for you. And then you point at the next target, etc. etc.
Also, it’s a butchered quote from Larry Niven’s Known Space books, referred to as the “Kzinti Lesson” - because the Kzinti thought humanity was unarmed and helpless until they discovered that humans are really good at improvising weapons.
Yup! Totally plausible - just more expensive and less repeatable. And harder to use against moving targets.