I did, too; I just don’t think it would work. The tricky part is suspending a rocket motor in the planet’s atmosphere, then applying thrust to it in a downwards direction. All that would happen is that the rocket would plunge into the planet.
It’s completely ridiculous. But Niven’s explanation is that
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the residual hot (from the fusion reaction that powers the motor) hydrogen compounds in the motor would help keep it suspended high in the atmosphere, and when it started to cool, the fusion lasers would heat it back up to a steady state just shy of actual fusion, maintaining buoyancy
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the downward drive of the motor (the upward exhaust would push it down toward the planet) would eventually cause the motor to run into the highly compressed lower atmosphere of the planet, which would push back on the motor, preventing it from diving any deeper
Both seem dubious to me, but then IANA Physicist. (Neither was Niven.)
The version I saw mentioned more recently in the webcomic Schlock Mercenary used a double ended “candle”, with a fusion drive at both ends.
He also pointed out the “shoving a candle up Uranus” joke, which I’d somehow missed.