I'm not Dave, but I think what he means is that the practice of the brahmaviharas are not limited to Buddhists. Every (sensible) religion promotes kindness, etc in various ways.
See Tolerance and Diversityby Bhikkhu Bodhi:
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So other paths will be conducive to many aspects of the eightfold path. However, the crucial insights into not-self are required for complete liberation:To the extent that a religion proposes sound ethical principles and can promote to some degree the development of wholesome qualities such as love, generosity, detachment and compassion, it will merit in this respect the approbation of Buddhists. These principles advocated by outside religious systems will also conduce to rebirth in the realms of bliss — the heavens and the divine abodes. Buddhism by no means claims to have unique access to these realms, but holds that the paths that lead to them have been articulated, with varying degrees of clarity, in many of the great spiritual traditions of humanity. While the Buddhist will disagree with the belief structures of other religions to the extent that they deviate from the Buddha's Dhamma, he will respect them to the extent that they enjoin virtues and standards of conduct that promote spiritual development and the harmonious integration of human beings with each other and with the world.
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To get that insight requires development of all the factors of the Buddha's Path...12. "Though certain recluses and brahmans claim to propound the full understanding of all kinds of clinging... they describe the full understanding of clinging to sensual pleasures, clinging to views, and clinging to rules and observances without describing the full understanding of clinging to a doctrine of self. They do not understand one instance... therefore they describe only the full understanding of clinging to sensual pleasures, clinging to views, and clinging to rules and observances without describing the full understanding of clinging to a doctrine of self.
- BB: This passage clearly indicates that the critical differentiating factor of the Buddha's Dhamma is its "full understanding of clinging to a doctrine of self." This means, in effect, that the Buddha alone is able to show how to overcome all views of self by developing penetration into the truth of non-self (anatta).