And did he really use the word “millionaires” instead of “billionaires?”

He was referring to a specific set of millionaires. The full quote is:

Since Biden’s disastrous debate performance on June 27, various Democratic donors have been sounding the alarm on whether he can beat Trump in November.

“I’m getting so frustrated by the elites in the party” calling for him to step down, the president fumed on MSNBC. “I don’t care what the millionaires think.”