If the Premium Sub offers something like +5% att in Duty Finder content, then fine complain, but it's not.
Pay2Win is the concept that without doing anything in-game you can buy your way to the best gear or gear that can't be obtained by other means.
Basically if I'm Freeplay Johnny and I play 2-5hrs a day and work hard to get that best gear that took me a month to get and then PayWinner Alex comes along and gives 50 dollars to get gear that is way better than mine, that's pay2win.
Freeplay Johnny has the best sword he can get in the game, it does +100 ATT and gives him +5 accuracy and that's it. Meanwhile, PayWinner Alex has a sword that granted him 120 ATT, +50% ACC+ and when he deals dmg with the sword it restores his HP. That's what pay2win means, it's unfair advantages that make the PVE/PVP mechanics of the game easier for the other player that pays more.
The "advantages" that the premium app offers are ease of access, having the prem app doesn't increase the odds of getting a minion through retainer by 50%, it doesn't automatically reduce the timer on all retainer mission by 50%, it just allows you to access said content through the phone without needing to log in. The coins are a way to limit the use of the app and honestly for those who think being able to change the price of an item you sell through the app at the cost of a coin is an advantage. . . it's not. Appy Jones paid a coin to change the price of his item to the lowest in hopes of it selling but since Online Jim is on he can just change it as much as he pleases.
When it comes to the logic of using said app to change the sell price of an item, I doubt you will see a lot of people using the app to undercut just to make gil. You will mainly see people using this app when they are looking for an item they want to buy, that minion that is rarely on sale but they see it during their lunch break, they can now grab it without worrying if it will be sold by the time they get home.