I think that this is a misunderstanding. The (first) vow is generally along the lines of: "Sentient beings are numberless, I vow to liberate them all". I've not come across Mahayana practitioners who are "holding back" (though they may well exist...).
I think that the problem we have is that the way the question is posed, and has largely been discussed in this thread, is based on different world views from the world view expressed by the Bhodhisattva vows. The question (and budo's post) emphasises the separateness of beings.
Mahayana practitioners will, on the other hand, emphasise the interconnection of beings, and that the not-self characteristic means that the separateness is an illusion.
Interconnectedness is not strongly emphasised in the Pali Suttas, but of course not-self, and the illusory nature of "beings" is:
And there are hints in suttas such as this:“Why now do you assume ‘a being’?
Mara, is that your speculative view?
This is a heap of sheer formations:
Here no being is found.
“Just as, with an assemblage of parts,
The word ‘chariot’ is used,
So, when the aggregates exist,
There is the convention ‘a being.’
“It’s only suffering that comes to be,
Suffering that stands and falls away.
Nothing but suffering comes to be,
Nothing but suffering ceases.”
https://suttacentral.net/sn5.10/en/bodhi#sc6
In cartoon form we could characterise two world-view extremes. [I'm trying to present these very simply, so please don't focus on the shortcomings of such a simplistic division!]“What, bhikkhu, is the earth element? The earth element may be either internal or external. What is the internal earth element? Whatever internally, belonging to oneself, is solid, solidified, and clung-to, that is, head-hairs, body-hairs, nails, teeth, skin, flesh, sinews, bones, bone-marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, contents of the stomach, feces, or whatever else internally, belonging to oneself, is solid, solidified, and clung-to: this is called the internal earth element. Now both the internal earth element and the external earth element are simply earth element. And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’ When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the earth element and makes the mind dispassionate towards the earth element.
https://suttacentral.net/mn140/en/bodhi#sc21
- Beings are completely separate and the teachings are about those beings individually liberating themselves by realising their not-self nature.
- Beings are not separate, the individuality is an illusion. Realising this interconnectedness is the realisation of not-self.
It's not my intention to defend the logic of either of these cartoon views, but I don't think that it's helpful to use one world view to criticise the practices that emerge from another world view. Mahayana practitioners criticising Theravada for being "selfish", or Theravada practitioners criticising Mahayana for "interfering" seem to be assuming these different world views and talking past each other. Better, in my opinion, to understand the world views and accept or reject at that level.
I would, however, like to make a final comment on this idea of "interference". There seems to be an assumption that helping other beings means "preaching" to them. That seems to me to be a gross oversimplification, and it would seem odd to criticise any Buddhist practitioners for developing generosity, kindness, and compassion, doing what they can to help others. Those attributes are certainly supported by the early texts...
Mike