Srivijaya already had colonies on Madagascar
in OTL. To this day the Austronesian language "
Malagasy" is dominant.
Nice try. The link you gave says nothing about Srivijayan colonies because there weren't any.
The Malagasy language is indeed Austronesian. It's a massively widespread lanugage family. The languages of the Malay Archipelago, Fiji, New Zealand, Easter Island, Hawaii, Taiwan and so forth are all Austronesian. That doesn't mean that Srivijaya colonised them. It just means that the Austronesians (Malays, Polynesians etc.) were very much maritime seafaring cultures. If we're going by your logic, all the following were part of Srivijaya.
Which they weren't because language distribution does not necessarily indicate political control.
Furthermore the Malagasy language is closely related to the dialect of Sulawesi, not that of Sumatra, which is what you would expect if they were somehow descended from nonexistent Srivijayan colonies.
I explained all this in the other thread you posted.