The article says:
Defense lawyers say the evidence does not show a celebration of violence.
Which implies that the prosecution claimed exactly that, or at least that the defense anticipates the prosecution to claim something along those lines.
The article also says [bold italic emphasis mine]:
Defense lawyers for Rhodes have previously addressed the dinner, saying that prosecutors’ theory of the episode is incorrect. Rhodes’ lawyers said in a court filing the dinner is evidence that the militia leader wasn’t working to foment a revolution.
“The conditions would never be better. Yet, Rhodes and the others left the Capitol grounds and went to Olive Garden for dinner,” lawyers for Rhodes wrote. “The answer is quite simple: because stopping the certification, overthrowing the government, was not Rhodes’ intent. It was not the Oath Keepers’ intent.”
So, from the CNN article, it seems that the prosecution interprets this as celebrating a successful attack whereas the defense argues that leaving the Capitol to go for dinner instead of continuing the attack proves that they didn't want to overthrow the government.
Why does it matter where they go for dinner and how much they spend?
It doesn't matter where, exactly they went, which restaurant they went to, and that it was an Olive Garden.
For the prosecution, what matters is that they appeared to celebrate, for the defense, what matters is that they left the Capitol.
It also doesn't matter how much, exactly, they spent. What matters is that the amount of money can be used as a proxy (by the prosecution at least) for how many people attended the "celebration", how long it lasted, and how much value was placed on the "celebration". (In general, people spend more money when they are celebrating something than when they are just getting a bite because they are hungry.)
So, in short:
- Prosecution: Celebrating how close they came to overthrowing the government proves that was their goal.
- Defense: Going for dinner when they had the chance to finish the job proves it wasn't their goal.