If I were to say something like "That's false, coming from someone who has lived in China for 3 years", then does the 'someone' in the sentence necessarily refer to myself?
"Coming from someone..." does not necessarily have to refer to the speaker (first person) Although, it is always singular. Consider:
"I understand why that would seem valid, coming from someone with a mother like yours." (Second person)
"Fashion critiques are hard to take seriously, coming from someone who dresses like Zelda." (Third person)