Similarly, proprietary software can be secure despite being closed-source.
That depends entirely on your threat model and the kind of relationship you have with the software vendor. Software might be proprietary and closed source but e.g. you might be the only customer and did get to engage an auditor which could see the source code. Or it might be off-the-shelf software made in a country trying to spy on your company or country. In some of those cases it literally can not be secure for your threat model.