I went and found the original article at the Athletic.
By Simon Johnson
https://www.google.com/amp/s/theathletic.com/4189842/2023/02/14/koulibaly-Chelsea-champions-league/%3famp=1
The tweeter misworded their tweet, the anonymous spurces were NOT The Athletic writers
Johnson says this:
Sources close to Koulibaly, who spoke to The Athletic anonymously to protect their relationships, explain nothing untoward is to blame, just a scenario that frequently occurs with new signings.
After spending eight years at Napoli, building an understanding with his team-mates and feeling comfortable in the city, Koulibaly has found it difficult to adjust to a new working environment.
He is still getting used to the pace of the Premier League and this hasn’t been helped by Chelsea’s patchy form. The team also keeps changing, which makes it more difficult to develop an understanding with team-mates.
And though Badiashile has played well under the same circumstances, the pressure and expectation on him aren’t as great as they are on Koulibaly.
After helping Chelsea secure a 2-0 victory over Bournemouth on December 27 (the club’s seventh win of the league season from 15 matches), Koulibaly admitted he had not been performing as well as expected. That match was Chelsea’s first since the World Cup, where Koulibaly had played for Senegal. The break from club football had given him a chance to reflect.
Koulibaly said: “I took some moments to think about everything — about football, about Chelsea — and to come with more motivation because the impact with this team didn’t come like I wanted.
snip
But Koulibaly does not come across as someone who is ready to give up on making his Chelsea move a success. According to the sources referred to above, he is enjoying being around London and is in the process of buying a new house in the city.