Where do these rumors come from? How could sannyas be post Buddhist when Buddha himself was a sannyasin??
First, sannyas is spoken of in the Vedas. See the Brahma Sutras verses 3.4.17-20 and 3.4.49 which has many vedic references to Sannyas.
Chandogya Upanishad 2.23.1-2 (this and subsequent verses are translated by Swami Nikhilananda:
There are three branches of duty; sacrifice, study, and charity are the first...All these attain to the worlds of the virtuous; but only one who is firmly established in Brahman attains immortality.
And the Brhadaranyaka Upanishad 4.4.22:
Desiring this world (the Self) alone monks renounce their homes.
See also Mundaka Upanishad 1.2.11 and Chandogya 5.10.1
Second Moksha. The whole point of the Upanishads is to teach moksha. Saying there is no moksha in the Upanishads is like the man who gets up after hearing a recitation of the Ramayana and asks "Excuse me, what was the name of Rama's wife?"
Kena Upanishad II. 5:
If a man knows Atman here, he then attains the true goal of life. If he does not know It here, a great destruction awaits him.
See also Brhadaranyaka Upanishad I. iv. 10
And Mundaka Upanishad III. ii. 9:
He who knows the Supreme Brahman verily becomes Brahman.
Brhadaranyaka Upanishad IV. iv. 6-7:
But the man who does not desire is not reborn. Of him who is without desires, who is free from desires, the objects of whose desire have been attained, and to whom all objects of desire are but the Self--the pranas do not depart. Being but Brahman, he is merged in Brahman, Regarding this there is this verse: 'When all the desires that dwell in his heart are gone, then he, having been mortal, becomes immortal and attains Brahman in this very body.' Just as the lifeless slough of a snake is cast off and lies on an ant-hill, so does this body lie. Then the self becomes disembodied and immortal; it becomes the Supreme Brahman, the Light.
and Mundaka Upanishad III. ii. 8:
As flowing rivers disappear in the sea, losing their names and forms, so a wise man, freed from name and form, attains the Purusha, who is greater than great.
Finally kaivalya. There is a Upanishad called the Kaivalya Upanishad which is part of the Atharva Veda. It is a short Upanishad of only 24 verses. It speaks of sannyas, kavilya, and moksha. Verses 2 and 3 say (Swami Madhavananda translator):
And to him, the Grandsire (Brahma) said: "Know (this) by means of faith, devotion, and meditation. Not by work, nor by progeny, nor by wealth, but by renunciation, some attain immortality.
Higher than heaven, seated in the cave (Buddhi) that shines, (which) the self-controlled attain--the self-controlled, who being of pure minds have well ascertained the Reality, by the knowledge of Vedanta, and through Sannyasa or renunciation. In the sphere of Brahma, at the time of cosmic dissolution, they get all liberated from the highest (apparent) immortality of the manifested universe.