Is using synthetic datasets in sciences the same as using fake data?
Synthetic data and fabricated data are different. If you want to use synthetic data, you need to be transparent about how the data is generated and not tailor the synthetic data to support your hypothesis. Fabrication would be false data that is designed to support your hypothesis.
Remember that any statistical analysis will be a reflection of the process used to generate the data. It might be that the trend in the synthetic data reflects real data, but you would have to make that case by examining how the synthetic data is generated.