The only time I remember of him playing chess was when he beat Oppenheimer
Judging only by that game, neither was anywhere near 1800. More like 1200 to 1400 than 1200 to 1500. Having been a club player/ first team captain for many years, I can more or less recognise their level of expertise, which seemed approximately equal. Well, maybe Einstein is 1400 maximum and lower boards club second team material, which isn't bad for a beginner, and Oppenheimer 1000 or 1100.
The only time I remember of him playing chess was when he beat Oppenheimer