It doesn't contradict what I'm saying. If you mean the Romanian case, the ultimate resolution was that 1) an employee has an expectation of privacy for personal communication on personal channels on a corporate machine, and 2) if an employee's personal internet usage is going to be monitored on a corporate machine, they must know before the monitoring begins. The ECHR didn't have any problem with the monitoring in general; what they ruled against was the legality of personal internet usage monitoring before the employee knew about it and agreed.
In the ECHR case, the employee's personal communications over an instant messenger were being monitored just because they were happening on the employer's machine, and without the employee knowing ahead of time. That's the no-no. But Slack is not a personal communications medium; it's maintained and administrated directly by an employer for the explicit purpose of work-related communication. In the context of the ECHR case, Slack doesn't qualify.
Circling back to Slack and the GDPR: as a direct result of the GDPR, Slack now needs to align their desire for full employee auditability with full data transparency. There's a tension between competing regulations, but there's no contradiction here.
Slack can be used for e.g. union organizing, which an employer may not read either, and "they must know before the monitoring begins" is obviously not given here.
This change is precisely because of regulations like GDPR, among others.