That is true, but that is not proof that the F-15 came from the MIG-25 in any way. You have to have some knowledge of how aircrafts are designed and how long it takes to field an aircraft, from conception to mass production.
The average time span from conception to mass production is about 10 yrs. But instead of mass production, let us use first flight as the first flight is usually the final proof of concept and to convince the government that the aircraft is beyond airworthy.
The MIG-25's first fight was in Mar 1964. The F-15's first flight was in Jul 1972. Essentially, both aircrafts were under parallel development. Just because the MIG-25's first flight was earlier than the F-15's, that does not mean we knew about it
EXACTLY on March 1964.
First flights usually do not have 'Top Gun' style maneuvers. First flights are always to prove the airframe's airworthiness and that it can perform basic maneuvers such as aileron rolls, touch and go, and so on. No one chases the aircraft to simulate combat. After the official first flight, the aircraft would be take apart, not completely, and examined for any distress. Any panels that are not in place, any airframe cracks, etc. From first flight, it can take several yrs towards mass production.
http://www.globalsecurity.org/military/world/russia/mig-25.htm
The MIG-25 was finally 'introduced', meaning make available for the military, in 1970. We knew about the MIG-25 before 1970, but not in 1964. As much as I believe in US technological prowess, there is no way the US could have developed the F-15 from the MIG-25 in a couple of yrs -- just from a 2D satellite photo