Even if there is only one aspect of online play that requires the use of a smartphone app it's a shitty thing. The gaming functions of a handheld console should be able to be operated completely independent of any other personal electronic device.
So I hope this is just Nintendo doing its usual thing and utterly failing to clearly communicate this thing, rather than actually making a smartphone a compulsory companion accessory in order to access 100% of on the go gaming functionality.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix