there are the court filings and there is the interpretation. There was a group of cyber researchers hired by the government to investigate Russian cyber crime. That group had legal access to said data. So "infiltration" appears unlikely, they didn't have to infiltrate anything.
Even that this was on behalf of Clinton seems unproven. From the Wiki, bolding is mine:
In a December 2021 court filing, Sussmann's attorneys presented portions of two documents provided to them by Durham days earlier which they asserted undermined the indictment.
One document was a summary of an interview Durham's investigators conducted with Baker in June 2020 in which he did not say that Sussmann told him he was not there on behalf of any client, but rather that Baker had assumed it and that the issue never came up. A second document was a June 2019 Justice Department inspector general interview with Baker in which he said the Sussmann meeting "related to strange interactions that some number of people that were his clients, who were, he described as I recall it, sort of cybersecurity experts, had found."
The New York Times reported that the narrow charge against Sussmann was contained in a 27-page indictment that elaborated on activities of cybersecurity researchers who were not charged, including what their attorneys asserted were selected email excerpts that falsely portrayed them as not actually believing their claims. Trump and his supporters seized on that information to assert the Alfa-Bank matter was a hoax devised by Clinton supporters and so the
Trump-Russia investigation had been unjustified. Sussmann's attorneys told the court that the new evidence "underscores the baseless and unprecedented nature of this indictment" and asked that his trial date be moved from July to May 2022.
[12] A Durham prosecutor later asserted that subsequent to his 2019 and 2020 interviews, Baker "affirmed and then re-affirmed his now-clear recollection of the defendant’s false statement" after refreshing his memory with contemporaneous or near-contemporaneous notes.
[13]
Being a court filing, it is based on a sound good faith belief that is true and can be proven.