Does JMIR publish articles about products from a company when all authors are affiliated with the company?

This is a case-by-case decision made by the scientific editor. Generally we discourage "marketing" articles and only consider rigorous and self-critical evaluations (negative trials are welcome and raise less suspicions than a positive trials). The less subjective a study is, the higher the chance for acceptance. For example, qualitative studies may be easier to manipulate or more subject to bias than quantitative studies. Papers which are prepregistered (protocol published before data collection, see What is a Registered Report?) also have a higher chance of acceptance. The editor/reviewers may request access to the original data to verify the integrity of the analysis and interpretation.

For papers where all authors are affiliated with a company we highly recommend to add an author without COI (e.g. an academic), who can serve as guarantor for the integrity of the data.

In any case, disclose of all conflicts of interests (COIs) is mandatory, specifying if any of the authors are current or past employees with the company affiliated with the product in question, and/or whether any author has any equity stakes or other COIs.

See also:

How does JMIR define a Conflict of Interest (COI)?