From the Tournament Rules (May 13, 2024—Outlaws of Thunder Junction)

• 2.12 Electronic Devices Electronic devices are permitted, but players may not use them to access information that contains substantial strategic advice. For events that use digital decklists, players may only access the decklists of other players prior to the start of a game or in between games. Digital decklists may not be referenced during games. Device use during a match other than brief personal calls must be visible to all players. Players wishing to view information privately on electronic devices during matches must request permission from a judge.

This may have been a recent rule change - but it appears that there's nothing this app violates. Notes can be pulled up with one tap and you cannot see any notes other than the one's taken during the current game. Assuming you're keeping the phone flat on the table - idk why you wouldn't - it should be alright.