He did not "burn" bridges, the relationship was very positive up until the complaint. And he "left" the Uni only temporarily because of an administrative technicality. This is a common thing in German-speaking countries. Some universities have hard limits on the total terms of study. You can avoid racking up total terms by temporarily ex-matriculating yourself.
He also only retweeted the "racist" cartoon and did not think much of it. He was not even aware it was considered to be problematic. Something like this can easily happen to anyone. Especially, in the heated debate climate of the last year.
They university did not expel him, the advisor just cut off all contact. This is more a Kafkaesque situation where the university is exploiting a technicality to comply with Chinese pressure.
Also, according the professor, the only Chinese pressure was an email from a Chinese foreign student in Canada who felt offended by the cartoon. It seems like the initial email from the professor was just not worded carefully and had some speculations about visa repercussions and that is what the entire article is based on.
https://www.nzz.ch/english/swiss-phd-students-dismissal-spot...