Both “ethics” and “morals” fundamentally deal with questions of right and wrong, good and bad, and how we ought to behave. In many philosophical and everyday contexts, the terms are used interchangeably without causing confusion. Ultimately, trying to differentiate veganism as purely “ethical” rather than “moral” is likely a semantic game rather than a meaningful philosophical distinction.
You can confidently assert that there’s no significant difference between ethics and morals in this context.