Actually, it is not!
Mahabharata Adi Parva, Arjuna Vanavasa Parva Chapter 217 describes Arjuna's pilgrimage route.
Getting down from the Himalayas, he went eastward. He visited many famous pilgrimage sites in Anga (Bihar-Jharkhand) and Vanga (Bengal). For example, he went to Gaya kshetra. Then he went to Kalinga (Odisha).
From Kalinga, he went to Mahendra ...
Mt. Semeru is located in Indonesia.(https://en.wikipedia.org/wiki/Semeru) India and Indonesia have a lot of common culture. And Indonesia was like an extension of Indian kingdoms before the Dutch colonized Indonesia and British colonized India. Hinduism was the primary religion in Indonesia before Islam became dominant.
There is no way to prove or disprove such claims. You have mentioned the Sarasvati hymn in Rig Veda VI.61. There is another mention of Sarasvati in Book 7.
This stream Sarasvati with fostering current comes forth, our sure
defence, our fort of iron, as on a car, the flood flows on, surpassing
in majesty and might all other waters;
Rig Veda VII.95.1
Sita was likely born somewhere in jungles of Himalayas, perhaps close to Kailash mountain.
The story of Birth of Sita starts with that of Vedvati. When Ravan received strong boons from Shiva, he went around the earth fighting many Kings and destroying many kingdoms. Once he was roaming around in jungles of Himalayas, that's when he spotted beautiful Vedvati.