When can it be called transfer learning?
Good question but rather besides the point.
What I mean is that even though estimating european house prices can be considered the same task (as estimating american house prices), nevertheless in this example the NN was re-purposed using transfer learning methods from another task.
Whether one would like to call the european house prices as same or related task, is not so much the point here (which can become pedantic), but rather if transfer learning methods were used and how succesfull they were.
IMHO