If one makes truth claims, one carries the burden of demonstration.
And, unfortunately, language is a terribly convenient tool for engaging in both deception and self deception. So to prohibit deception as well as self-deception, we must rely on a demonstration of knowledge of construction of terms, not just a knowledge of the use of terms. Just as we must rely upon the demonstration of internal consistency using logic, and external correspondence using tests.
This means that if you make a truth claim using platonic language, you are not demonstrating knowledge of construction.
And therefore is it is not possible to make truth claims under platonism.
You are claiming truth which you cannot demonstrate the knowledge to claim.
Which is unethical.