If it feels problematic, it's probably because you are missing the fact that there is a control and treatment group of this experiment to control for what you are saying.
It was stated that the treatment group did the assigmment twice, the first evening without, the second evening with electrical stimulation. Without evidence to the contrary, the memory improvement could be due to consolidation of the stimulus received instead of 'new' memories being stored better.
Although it could be much clearer, the article doesn't actually specify an order. Half the participants did the stimulation night first, per the paper:
>participants were tested during two experimental nights (order counterbalanced): an intervention night and an undisturbed night
