I think this is a statement found in many different traditions. Again, it is a statement obscure before awakening and clear after, and clear even through for instance the Big Mind process.
When there is an awakening to selflessness, it is Ground – Spirit, Buddha Mind, Divine Mind, emptiness, awakening to its own nature of no I anywhere. And this Ground is functionally connected to a particular human self.
From this awakening, it is clear that there are no beings, only the appearance of individual separate beings. There is only Ground (somehow separately) functionally connected to all the different beings, and in many cases temporarily identified with and as a sentient being. It is only an appearance, a temporary experience, a temporary misidentification, however real it may seem at the time.
For Ground still identified as a human being, it will appear as if another human being awakened. Yet from the view of awakened Ground, no being every existed – at least not as separate or individual in any way.
To speak in a very approximate way, we can say that in the absolute, there is no room for compassion. Big Mind is cool, detached, seeing itself everywhere, seeing no real (separate, individual) beings anywhere. Yet, moving towards the relative and Big Heart, there is tremendous compassion for all the suffering experienced, even if it is just Ground temporarily misidentified, experiencing itself, as a sentient being.
Speaking more accurately, we can say…
Ground awakened to its own nature, functionally connected with a human self, will naturally manifest compassion when this human self encounters Ground misidentified as a sentient being, manifesting suffering.