This is just a flat out wrong statement.
This is his original paper on Special Relativity, published in Annalen der Physik #17 1905 You will find no mention of any thought experiment, but, you will find the math to back up his radical (at the time) new ideas on spacetime. You will also note that he provides the math for the changes in Maxwell's equations.
Here you will find his original paper on General Relativity (Published in Annalen der Physik #49 1916 and also read to the Prussian Academy of Sciences in November of 1915. Note again, no thought experiments, but plenty of math.
He used the though experiments to try and describe his theories in a non-mathematical way, to the general public. Perhaps you are thinking about his book "Relativity the Special and the General Theory" In this, he uses thought experiments to explain the two. Note though, that this book was published in 1916. After he had already published both papers introducing his theories.
But, be very sure about this, he had the mathematics to back up his "thought experiment" descriptions of his theories, before using the thought experiments to describe them. Many amateurs make the mistake of coming up with descriptions, without understanding, or expecting others to do the math.