There's an option in Sympli Sketch plugin that lets you modify the default behaviour of selecting artboards for export on the current page only when you invoke the plugin without any artboards or layers preselected on the canvas.
To access this option, open the plugin menu (look for the gear ⚙️ icon at the top right of the plugin window), then go to Advanced > When no artboards are selected... menu and choose the behaviour you expect:
Note: if you don't see this option, make sure your Sympli plugin is up-to-date.
Export all artboards from the current page only (Default option)
This is a default behavior of the Sympli plugin: whenever you invoke the plugin window, and if you don't have anything selected in the current document, Sympli will mark all artboards on the current page for export.
Export all artboards from every page except "Symbols"
With this behavour, whenever you invoke the plugin window, and in case you don't have anything selected in the current document, Sympli will mark all artboards from all pages (except "Symbols") of the current document for export.