Yes, commentary is both standard and frequent. Even from the 3rd century BC, librarians at the great libraries were scholars in their own right (most notably at the Library of Alexandria). And remember that all documents at this time were hand copied. That leaves every copy of a text open for error, and if it's copied from a bad text, then errors will perpetuate. A regular part of a librarian's work was to parse ancient texts for 'accuracy.' That is, they'd add bits that they felt needed adding, and remove parts that didn't feel true to the author. Most of these alterations would be noted in the margins with detailed notes, known as scholia. Scholars that were engaged in this pursuit were/are known as scholiasts.

Analysis and reanalysis of texts was persistent, and still is. It was the only way to ensure a cohesive 'true' text without overeager librarians adding hundreds of lines to an earlier text. And the 'true' text is still revisited regularly, with the Oxford Classical Texts and Teubner editions being the most highly regarded. And commentary for additions, subtractions, and alterations is still included.