In modern Indian practice vajrasana usually names a simple kneeling seat: shins on the floor, sitting back on the heels, spine upright. It is the posture recommended in a great deal of popular instruction as a comfortable seat for breathing practice, and it is not the arrangement the Gheranda Samhita describes.
The atlas records the classical description and notes the modern usage as a separate fact. That approach is applied consistently across this atlas, because the alternative, quietly harmonising the two, would make the source say something it does not say and would remove the most informative thing about the entry.
It is worth adding that the direction of change here is unlike the camel or the raised posture, where a demanding modern shape took a classical name. In this case a simpler seat took the name of a firmer, lower and more specific one. There is no single pattern to how the posture vocabulary shifted, which is itself a finding: names moved in both directions, and they moved through teaching lineages, translations and printed manuals over several centuries. Mark Singleton's Yoga Body sets out the twentieth century stage of that process in documentary detail, and Mallinson and Singleton's Roots of Yoga supplies the primary descriptions against which any comparison has to be made.