Yes, the difference between filibustering and grandstanding plays a part. Equally important is the fact that Cruz's theatrics are frustrating members of his own party. But, part of the disparity in coverage is due to the fact that the mainstream media, generally speaking, don't admire Cruz the way they admired Davis -- or rather, they admire him only insofar as he makes for tragicomic theater, whereas they admired her on the merits.
Now, it's worth noting that for all the excitement about Wendy Davis, it was largely an online, activist to-do. That's not to diminish it. It just didn't have even a fraction of the coverage as the Cruz situation.
But this is his analysis. The absence of any rational difference between the two events means that media bias must be at work. Except at Byers himself notes, Cruz's wild grandstanding absent any ability to filibuster and the fact that his 'theatrics' and general obnoxiousness have made him reviled by a weighty number of his Republican colleagues in the Senate. So it's not just liberal media bias. It's also Republican media bias. Or just Republican bias. The same way virtually all Cruz's classmates over almost a decade were biased against Ted.
These sorts of media bias stories are almost always rooted in laziness, just picking up on the self-interested griping of the supposedly aggrieved party, or most troublingly, simple derp.
You can't just assert bias because it's an obvious or simple column to write. You need some actual analysis or argument that might make it true. Not all things are equal. Some trees are tall and others short. Some comics are funny and others not. Noting these differences are not always bias. Sometimes they are merely noting differences between things that are different. Or to put it in another way, there is a thing called reality and it is relevant to how we report on things.
In this case, the fact that a substantial number of Cruz's partisan colleagues believe he is a self-promoting jackass - probably a good half of them in the Senate - is a good initial sign that something more than bias may be at work.