A transformer is trained on millions of elementary geometry theorems and used as brute search for a proof, which because of the elementary geometry context has both a necessarily elementary structure and can be easily symbolically judged as true or false. When the brute search fails, an extra geometric construction is randomly added (like adding a midpoint of a certain line) to see if brute search using that extra raw material might work. [edit: as corrected by Imnimo, I got this backwards - the brute search is just pure brute search, the transformer is used to predict which extra geometric construction to add]
Also (not mentioned in the blog post) the actual problem statements had to be modified/adapted, e.g. the actual problem statement "Let AH1, BH2 and CH3 be the altitudes of a triangle ABC. The incircle W of triangle ABC touches the sides BC, CA and AB at T1, T2 and T3, respectively. Consider the symmetric images of the lines H1H2, H2H3, and H3H1 with respect to the lines T1T2, T2T3, and T3T1. Prove that these images form a triangle whose vertices lie on W." had to be changed to "Let ABC be a triangle. Define point I such that AI is the bisector of angle BAC and CI is the bisector of angle ACB. Define point T1 as the foot of I on line BC. Define T2 as the foot of I on line AC. Define point T3 as the foot of I on line AB. Define point H1 as the foot of A on line BC. Define point H2 as the foot of B on line AC. Define point H3 as the foot of C on line AB. Define point X1 as the intersection of circles (T1,H1) and (T2,H1). Define point X2 as the intersection of circles (T1,H2) and (T2,H2). Define point Y2 as the intersection of circles (T2,H2) and (T3,H2). Define point Y3 as the intersection of circles (T2,H3) and (T3,H3). Define point Z as the intersection of lines X1X2 and Y2Y3. Prove that T1I=IZ."