It's explained here. The principle is to schock you to make you remember.
So, the first time you pass this exercise, you think: "What? two ten means twenty? Oh, that should that they multiply, and that "puluh" after another number is rather the mark of the teens that a real number, like two-teen.
You see their method is effective, now you've understood and remembered it.
We don't know if it's only a coincidence, officially it is, but, because there is not only one isolated coincidence, but several ones, and because the Austronesian origines are not very well known, despite the official linguistic theories, there's room for doubts and alternative theories.
The actual hypothesis is that Austronesian languages are a separate family from Indo-European ones, but who know, linguistic science will maybe make some progress and find common ancestors. For people who know for instance Hindi and Indonesian, there are really many of this coincidences, and I exclude those the words that have been "traded" by the mean of the Arabic language and culture.
There are also some similitaries in scripts:
Yes, that's true that weird coincidences happen too, the Mbararam "dog" is a classical linguistic textbook case (false cognate):
one PIE: sem "one united", Austro: sa Indonesian: se-
two PIE: du̯oe̯, Austro: dusa, Indo: dua (from root duha)
three PIE: trei̯es, Austro: telu. Hindi: tiin. Indo: triga.
That's not very easy to compare the vocabulary, because Indonesian have a lot of loan words, and most are from these origins:
- Arabic, Persian (not the same family, but Arabic borrowed from Persian, and gave the words in Indonesian)
- Portuguese, Spanish
- Dutch, French
And more recently, English.
For instance, the word for "animal" in Indonesian is "hewan", and in Hindi, it's "haivan", they both come from Persan حیوان (hayawān), it was brough by the mean of the Arabic language.
Indonesian first incorporated deeply many Sanskrit-Hindi-root words because of the religion, so it's probably hard to know what has been borrowed and what is from other roots, and if this root is really separated.
For the NativLang channel about linguistics, it's here: