Non-inferior is not “just as effective.” That would require an equivalence study to conclude, rather than a non-inferiority study.

Rather, a positive non-inferiority like this best interpreted as “aspirin is tolerably close to low-molecular weight heparin.”

Where the researchers have defined what “tolerably close” means. It could very well be 10% worse than low-molecular weight heparin. But if the non-inferiority margin was set to 15%, the study would conclude that aspirin was non-inferior.

You can’t interpret non-inferiority study results without knowing the non-inferiority margin and how it was derived and if it seems to be a reasonable amount or not.