From that response, it is quite clear that you do not know what the term SPAG means. There is no harm or shame in that; it is not an acronym that appears often so it is very understandable why someone would be unaware of what it means.
This would have been a perfect opportunity for you to ask median for clarification (something like asking 'What does SPAG mean?'). Instead you opted to assume that you know the correct definition and just roll with it. Or perhaps you 'just had faith' that you knew what it meant.
Or I looked it up in 2 or 3 places. What's your point again?
Based on all of the responses and questions you've received from median, you're telling me you honestly assumed he was slamming your grammar?
The point is just because you think
you know something doesn't mean you actually do