Denying it confirms the intent behind the move was malicious. Wether it was done by him or anyone else, so this pretty much confirms it as an attempted attack on the US by China or at worst, a single chinese person.
In the article :
“Liu stated that he intentionally hid the samples in his backpack because he knew there were restrictions on the importation of the materials,” the complaint says. “Liu confirmed that he had intentionally put the samples in a wad of tissues so CBP Officers would be less likely to find and confiscate them, and he could continue his research in the United States.”
"But an examination of Liu’s and Jian’s electronic devices uncovered WeChat messages from 2022 in which they discussed smuggling seeds into the United States."