very common scam in Bangkok, they are a salesman/tout. They always have a relative in australia. Never heard about the lunch invite, but I am sure this conversation was going to end up you being led to a shop, for overpriced goods, which they will get a commission. If you try to say you want to go somewhere else, they will tell you its closed for government function or something like that.
I have lost count how many times they tried to hit me up with this one. Its amazing that I was 20 years between ttrips to Thailand and the same scam continues, word for word.
The biggest clue to any scam overseas. if someone approaches you to strike up a conversation under pretence of idle chat, practicing english, its a scam. Normal people dont do that.