No. La Berge showed that lucid dreamers can reach out to their external environment (dreamer->experimenter), he did not show that they can react to external inputs (experimenter->dreamer).

I read the book when it came out so perhaps my memory is faulty, but I was pretty sure they tried different mechanisms for prompting the lucid dream.

One of them was embedding LEDs in a sleep mask. When their waves showed they were in REM sleep, the experimenters would flash the LEDs as a prompt to the dreamer to interact. They would respond either with a prepared eye left/right pattern or by clenching their fist. I don't recall asking them to do higher level thinking, such as simple subtraction problems, though.

Right, in some of these attempts cues are learned before the participants fall asleep, so that they react to it while dreaming. One could argue that the distinction is subtle, and La Berge could've found the results these groups are showing today, had he ran the experiment. Nonetheless, experimenter->dreamer communication during sleep as suggested in this paper is AFAIK indeed new.