In times of plenty, there is a risk of losing sight of Hashem and abandoning Judaism. Sustaining us in plenty, means that even when things were good, and the Jewish people were not being forced into otherness by the evils around them, Hashem kept them together and faithful.
"Times of plenty" can be referring to the wealth of the nation that the Jews are living in, and not the wealth of the Jews themselves. Sometimes in history, the host nation would tax the Jews out of all that they owned, making the Jewish people poor. Despite the wealth around them, the Jewish people felt that they were mainly getting their sustenance from Hashem.
There are other ways of reading this as well.