I have made a little research, and I am not perfectly sure that the claim is correct. There have been much speculation who was King Ptolemy in the Letter of Aristeas. Graetz (1890) rules out Soter and Philadelphus, and attempts to prove that he must have been Philometor. Bernays (1856) had argued earlier something similar in his essay called Über das Phokylideische Gedicht. This is a work by Pseudo-Phocylides, which is using themes from the Proverbs that are only present in the Septuagint version (see this book), so the Septuagint must have already been translated by its creation. When discussing the matter of dating Bernays writes:
Bevor das alte Testament der griechisch redenden Welt zugänglich geworden, also vor den ersten Übersetzungsversuchen unter Ptolemäus Philadelphus, konnte schwerlich eine solche biblisch moralische Anthologie entstehen, und die Mitte des dritten Jahrhunderts vor Chr. wäre demnach als die äusserste mögliche Grenze rückwärts anzusehen.
Before the Old Testament became accessible to the Greek-speaking world, i.e. before the first attempts to translate it under Ptolemy Philadelphus, such a Biblical moral anthology could hardly have emerged, and the middle of the third century BC would therefore be seen as the extreme possible limit backwards.
So here he claims that the Septuagint was not translated before Philadelphus, therefore, I can hardly believe that he could have argued elsewhere that it was translated roughly a century earlier in the times of Lagus. Hence, I suspect that the reference to Bernays in Marcus' work is imprecise.