If Napoli is still trying to improve its position in the Serie A standings, of course Napoli must always consider winning and it would be very unreasonable if Napoli deliberately lost the Coppa Italia to a team lower than them. Apart from that, Napoli also cannot give an excuse if they deliberately lost just because they are still in the Champions League this season, because their fans will certainly be very disappointed if Napoli admits that they deliberately lost a certain match even though it was outside of the Champions League competition.
I think Walter gamble it to see if other players can do better just so that they don't have to depend on main squad that usually start to the match for Napoli bit they failed and when substitutions happened, it wasn't effective as he expected. Throughout the game, it was only Napoli that substituted in that match and even when they did, Frosinone 2 goals when Osimhen was in. Such a Ln embarrassment to Napoli because they couldn't use their chances to goals while Frosinone was able to give into every chance with goal.
Honestly, I didn't blame Walter, Napoli has not been stable in Seria A league, a team that went from top 1 last season is struggling to enter top 4 and not only that, the points they have gather this season is to low, imagine 14 points away from the top of the first place in the league table says a lot, they have lost more than 5 games to be where they are today, they need to do more if they want to come to Champions League next season.