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They weren’t trying to generate electricity in this experiment. They were trying to sustain a reaction. As you said in another comment, they are different problems.
Converting heat to electricity is a problem we already understand pretty well since we’ve been doing it basically the same way since the first power plant fired up. Sustaining a fusion reaction is a problem we’ve barely started figuring out.
Most fission plants transfer the heat away from the reactor before boiling water. The same can be done with fusion.
The main difference with fusion is you have to convert some of the released energy to heat first. Various elements have been proposed for this.