Okay, after diving into the depths of the Interwebs, most of the references I found to a Sumatran "Father-of-all-the-turtles" derived from Heuvelmans' book (the one the
Dragon #190 Sea Monsters article is based on).
The forteanzoology blog has a nice page on
The Father of all the Turtles which appears to contain a good roundup of reported sightings as well as a report on an
Expedition to Sumatra which mentions an ancient Mediterranean bestiary description of an island-mimicking "Asp-Turtle".
The book
Mythical Monsters (1886) by Charles Gould describes an eyewitness encounter of a
200 foot long dragon/turtle/sea monster.
Mythical Monsters is now in public domain, so you can read it on
Project Gutenberg or
Internet Archives. The book quote a newspaper correspondent as follows:
The monster seen by the Nestor, however, was probably one of the Chelonidæ, “the father of all the turtles,” as he is fitly called by the natives of Sumatra, who fully believe in his existence, and to whom he occasionally appears. Indeed, Baumgarten, in his Malaysien, published at Amsterdam in 1829, describes the monster, and estimates its length and breadth at one hundred and twenty and thirty cubits respectively, measurements which agree very nearly with those given by Captain Webster. Baumgarten adds that it is a general belief in Sumatra (vol. ii. p. 321, Ed. 1820), that whosoever sees him will die within the year. “This,” he says naively enough, “I have not been able to prove.”
As for the Sumatran/Indonesian legends that this creature's name is supposed to come from, I haven't had that much luck. I came across multiple references to giant turtles being associated with divinities. There's a myth, the
Tantu Pagelaran, about Vishnu turning himself into a giant turtle to help carry a mountain to Java, and another one about a turtle
turning itself into Kusu island to rescue some fishermen - an interesting reversal of the ancient European myth of a giant sea monster masquerading as an island and
drowning sailors who land on it.
The famous
Hoan Kiem turtle is given the honorific "great grandfather", but that's in Vietnam, not Sumatra. Interesting it is also connected with a deity -
Kim Qui the Golden Turtle God - or possibly it's a turle-shaped messenger/intermediary from heaven.
Finally, I came across a mention that there's a conversion of The Father of All The Turtles in the HERO Games Asian Bestiary, Volume One. Fortunately, I have that book! It stats the Turtle up as a regular animal (18 metres long, decent armour, extra reach from a long neck, nasty beak attack) with the option to add a power-suite to make it an immortal, intelligent "turtle god".