Do people need an ECAD license to look at my designs on AllSpice

No. AllSpice renders images of your native ECAD designs on the browser, making them accessible to all collaborators (electrical engineers or not) who have access to your AllSpice organization. Browser rendering applies to PCBs, schematics, and BOM files in CSV format.

However, to make design changes to your schematics or PCB files, you'll need an ECAD license. Think of AllSpice as having view and comment privileges only.