In lieu of a response from the committee, I will comment based on what I know about the USGA rules… As I understand it, the app is USGA compliant. The USGA rule states you can use a green reading map for reading putts as long as the scale is within limits, which I believe it is for the GHIN maps since it is a USGA product.
Also, you can use a map with a finer scale as long as the purpose is NOT reading greens for putts. For instance, there are some older Strackaline books out there with a finer scale that you can continue to use for approach shot strategy. Using them to read putts would be a rules violation.
Having never played this event I don’t know if they provide competitors with a “hard card”. The hard card outlines local rules for the tournament, such as use of rangefinders, carts, caddies, one ball rule, etc. If it doesn’t state anything specific about a rule then it is assumed USGA rule applies, which I would interpret to mean the GHIN app is acceptable.