In fairness, most European writers in the past paid little to no attention to folk beliefs or practices until the 16th century.

References to guising in the 1500s (not 1700s) may only be the first written reference - it does not tell us when the practice actually began.

Lastly, traditions can have more than one origin - they are often accumulated blends from multiple backgrounds. For example we know Christmas is obviously Christian and yet Christmas Trees are not.

Halloween's Celticity is highly romanticised and likely exaggerated but not entirely without basis.