Yes. It is a conspiracy. You are saying a vast amount of Buddhists, somewhere, somehow, are clogging up the numbers to make it look like there are more Buddhists than actuality.
For one, experts on SE Asia constantly emphasise the vast syncretism of the region. For another, the languages, scripts and cultural practices of SE Asia come predominantly from the Hindu polities that existed. If you read travellers and tourists' accounts of SE Asia and see documentaries like Peter Owen-Jones "Around the world in 80 Faiths" you see constant mention and appearance of street Brahmans preaching in the streets or selling images of Hindu gods. Heck, Hindu temples populate SE Asia even if many are in ruins now, as are many Buddhist temples anyway.
Syncretism is a big deal there, yes. By far the majority of people there are Buddhist, however, and the number of Hindus is accurate. That point means nothing.
Also, your “logic” about the scripts, languages, and cultural practices of the region coming from old Hindu polities (which is mostly true) meaning that there are far more Hindus than we think is ridiculous.
By the same logic, there are far more Roman Polytheists than we think in Europe, 10% even to draw a similarly arbitrary number as you did. Wow, I didn’t know those pesky Christians were suppressing the Roman Polytheists like that! After all, look around. The languages are derived from (gasp) Latin scripts! And the cultural practices can be traced back to Rome.
Oh, and the point about the constant mention of Brahmins in this one certain book and Hindu practices seem by tourists is completely anecdotal, not to mention irrelevant. Hindus can be seen and prominent without having a large number, just like many religions across the world, and of course there are majority-minority-religion provinces as in most countries with a varied religious history. But the numbers are still accurate, and you have put no proof forth to the contrary other than anecdotal and irrelevant “evidence” and the “because I said so” argument.
Also, the phenomenon of sectarian inflation is not an unique one. It occurs in Lebanon where Maronites continue to claim Christians make half the population when in fact it may be as low just 25% at this point, as well as in Latin America and China, where Protestant missionaries inflate the number of Protestants that there actually are.
So... the only recorded circumstances of what you’re saying about Buddhists doing
actually happening is by minority religions who are under at least some danger of persecution by the majority? Huh. You know, going by
actual trends shown by these, it seems there may be less Hindus than the numbers show, but under no circumstance
more. The SE Asian Buddhists are a majority religion obviously, and would have no reason to do this going by the given examples in the modern world. Hindus, however would. (Not that I believe that Hindus are bumping their numbers, I’m just showing how ridiculous your argument really is if this is the evidence and precedent for it.)
I have given actual graphs and charts and numbers in this argument, as well as historical arguments based in trends and modern day parallels. This is not to mention actually doing math and crunching numbers and finding your points completely false and numbers completely arbitrary. You have made an unsubstantiated claim, still have not substantiated it, and have instead fallen on anecdotal and irreleveant points to try to prop up the collapsing house of your claim with balsa sticks.