Issuing a PR stating that Mazars was selected as the auditor would be enough grounds for a lawsuit if Mazars is not engaged as the companies auditor.
In the audit world firms reprutations are very important and often is what gets then in the door to new clients, simply naming name in a PR if it were fake would be enough for Mazars to chime in and make ONCI retract that statement.
Mazars currently does not have any OTC clients, if this were fake I would think that Mazars would be quick to dispel that rumor