Yellen’s basically saying the Chinese are selling green energy products below cost, aka “dumping” without actually using the word, which would be an accusation of unethical conduct.
Foreign manufacturers, especially those with nation-state support, do “dumping” to grab market share and destroy local competition. The foreign companies eat the initial losses, but then they can raise prices after local companies are destroyed, acquired, or hopelessly behind and competition has been effectively eliminated.