,  this was what a 70yr black old man said to me after a  small encounter. just to put things in a better perspective I am also black in my 20s and this happened in London.

This man came to my shop, a drug store to obtain his medications and he asked how to use them. He asked if the two medicines can be used together I said yes (because they can be used together) and he further asked if they are the same " i said no they are not the same but complimentary" he looked at me and said "you are eurocentric" and he packed his medicine and worked off.

I know this old man might be loosing his faculties but can anyone deduce from what i have just described how he came to this conclusion.

Maybe because you said it was complimentary Maybe you didn't greet him properly

I think you mean "complementary" and not "complimentary." These two words have different meanings.

The old man in that story sounds confused. Things can complement/assist each other without being the same. In fact, to complement each other, they almost certainly have to be different.

you didn't say anything wrong, the old man maybe losing it