I'm not going to speak for Splashmouse and their specific preferences, but, as a general case, that's not a great assumption to work off of. Some folks don't mind if he or she or they is used for them, and some folks would like to be exclusively addressed with they/them because they don't feel he/him or she/her accurately reflects their identity. There may be folks who are fine with he or she pronouns even if they primarily use they/them, but if someone says they would like to be addressed as they/them, it's best to comply and use they/them for them unless they specifically mention that they don't mind he/him or she/her.
