[astrodust]

Citizenship doesn't matter, but the location of the user does.

If you're doing business with American users, defined as "people residing in or doing business within the borders of" the United States, then you're subject to the patent laws.

All a claimant needs to do is prove that this is the case and the lawsuit can proceed. Then you'll need to find representation in a particular court in Texas to defend yourself, or you'll have to settle out of court at your own considerable expense.