Dubois, 27, arrived at Wembley Stadium at around 20:15 BST - less than two hours before his scheduled ring walk.
He was stopped in the fifth round, losing his IBF heavyweight title as Usyk, 38, became a two-time undisputed world champion.
Dubois' trainer Don Charles told BBC Sport that he could not confirm every detail of the video but admitted that such a gathering had indeed taken place.
"I would like to change the word from party to gathering," Charles said via video call.
He explained that a similar event was held before Dubois' knockout victory over Anthony Joshua in September and that the approach was an attempt to replicate what worked then.
"It's not unusual," Charles said. "This gathering is the same gathering which was used for the AJ fight day preparation. A lot was made of Daniel's ring walk for that fight - how enthusiastic and confident he looked - and people were praising him."
There are unconfirmed reports that Dubois' father organised the gathering and that the fighter left during the event to go for a short drive.
"To put logic to it, the father mainly replicated the recipe that worked against AJ to hopefully get a victory last Saturday - and it did not work," Charles added.
"Consequently, there is now a big highlight on this party, which was a gathering."
It is also alleged that some members of Dubois' entourage were refused entry to Wembley, contributing to his late arrival.
Usyk, who had stopped Dubois in the ninth round of their 2023 meeting, floored him twice in the fifth round of their rematch, with Dubois unable to beat the count after the second knockdown.