1999 Volkswagen Golf gearbox crack

I have just brought this car and the gear change was a but stiff. I got some gear oil to to up but as it turns out I put it in the clutch inspection hole not the gear top up hole! As a result the clutch was slipping. I took it to the garage to have the clutch replaced. They saw the oil in the clutch so took the gearbox off and inspected it and found a hairline crack which they assumed was where the oil came from. Advised them that the oil was my fLut as it topped up in the wrong place but they are still insisting that I need a new gearbox. Is this true? As the oil in the clutch was from my mistake not the crack. They said they filled the crack with liquid metal sealant and fitted a new clutch but do I really need a new gearbox?