From the article:

“He may be sentenced to die in prison in connection with his efforts promoting liberty in China.”

Martyr doesn’t sound like overstatement if that happens.

It seems accurate and unsensational here

I think perhaps we've lionized the term martyr to mean too many things, but his actions seem in line with the dictionary definition

This man will spend the rest of his (possibly short?) life in prison for the crime of publishing ideas that the government didn't like. He chose to stay in hong kong defending the principles that mattered to him instead of abandoning his principles and fleeing to the UK, which was on option entirely open to him.

Explain why you think it's overstated.

Jimmy Lai is more courageous and principled than anyone you've probably ever met in your life. He'll die in prison for his belief that speech should be free in Hong Kong.