Buddhism has adapted to many different cultures but its doctrinal core has remained the same. The concepts of karma and rebirth were pretty much as foreign in China (and other East Asian countries) as they are to the West. Nevertheless the Chinese Buddhists accepted these ideas- they didn't toss them aside as Indian cultural baggage.
The Four Noble Truths have to be seriously re-worked without karma- actually they would mean something different. All the discussion on suffering, cause of suffering, and end of suffering relies on karma. The sutras go on and on about the different kinds of karma and the different kinds of rebirth they entail, which such exhaustive, flat-footed detail that a metaphorical interpretation is out of the question.
Belief determines practice. Most Buddhists do not just meditate on their breathing- there is a plethora of devotional practices in Buddhism which assume supernatural beliefs. Vajrayana Buddhism is especially dependent on these beliefs- without them, all the stuff about guru devotion, deity meditation, torma offerings, reincarnate lamas- it all falls apart. If Batchelor is right then all the great Buddhist teachers wasted vast amounts of precious time. Batchelor must be the real Buddha for our age.
I see it much differently than you do. If Batchelor is the real "Buddha of our age", than he is one among very many people who practice Buddhism without holding to supernatural belief.
Buddhism rejects materialism. Once you assume materialism you are not practicing Buddhism but some other philosophy. A key teaching of Buddhism is that the external world itself is a product of mind.
As for "very many", well, these views float mainly around Western self-described Buddhists, who are already a very small part of the population. Around the world Buddhists maintain belief in karma and rebirth.
Admittedly, Batchelor probably goes a bit far in trying to strip the Buddha from tradition and place him in a western mold.
But in What the Buddhist Taught
(page 11) author Walpola Rahula writes that after the Buddha explained kamma to his disciples, he followed with: "O bhikkhus, even if this view, which is so pure and so clear, if you cling to it, if you fondle it, if you treasure it, if you are attached to it, then you do not understand that the teaching is similar to a raft, which is for crossing over, and not for getting hold of." The belief that the Buddha of the Pali suttas is endorsing is a kind of working hypothesis: "Come and see." He encourages doubt, uncertainty, and skepticism regarding the teachings of traditions and the dogmas of "holy men" (in the Kalama Sutta, for instance) and teaches one how to investigate for oneself. Kamma is observable in the here and now, too. Thoughts and actions have repercussions, and one can learn to see this acutely through Buddhist practice without necessarily believing that it will ripple into another life in the future.
I hardly think that rebirth, for instance, must be blindly accepted for one to begin to practice - even seriously practice - Buddhism. Therefore, regarding the OP, I cannot see why an atheist can't call himself a Buddhist and practice Buddhism.
As for "very many", well, these views float mainly around Western self-described Buddhists, who are already a very small part of the population.
I have spent nearly as much time on Buddhist discussion forums as on here, and I suspect that the portion of the population who approach Buddhism in this way is bigger than you think. Sure, many of them are westerners, and that is because, as I mentioned above, westerners are filtering their understanding of Buddhism through their own understanding. If they seriouisly set out on the path, who knows what they'll discover through practice? And you don't have to call them "Buddhists" if it makes you feel better.