Well spotted! Our bad, I'm afraid: I'm fairly sure this is a mistake. As you point out, Rene Descartes was born in 1596, and Chaucer in around 1340, the two men lived in entirely different times - and Chaucer certainly didn't write to Descartes.
The writer means, in fact, Eustache Deschamps, a (not very well known) medieval French poet who in 1385 wrote to Chaucer, calling him 'great translator' and noble'.
Sorry for the confusion!