Question
Did Rama really call Buddha as a thief?
Answer
First of all, this translation translates the word buddah in the literal meaning of intellection, rather than the name of the founder of Buddhism:
It is an exact state of the case that a mere intellection deserves to be punished as it were a thief and know an atheist to be on par with a mere intellectual. Therefore he is the most suspectable and should be punished in the interest of the people. In no case should a wise man consort with an atheist.
Second of all, Buddha got refer to the previous Buddha incarnation that Vishnu took to lead the Asuras astray in Tripura, rather than to Vishnu's incarnation as Siddhartha Gautama. The story of Tripura happened before the Ramayana.
In any case, many scholars do think this reference to Buddhism is an anachronism and that this passage is an interpolation; here's what Gaurinath Shastri says on page 29 of this book:
Traces of Buddhism cannot be found in the Ramayana and the solitary where the Buddha is mentioned is believed to be an interpolation.
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