yea but you can’t install GrapheneOS on Fairphones
for some people the environmental impact of using Fairphones or older Google Pixel (which CalyxOS supports longer) is more important than using a slightly more secure custom rom

for anyone looking for another custom rom: CalyxOS does support Fairphone 4 (and 5 later) while locking the bootloader with custom keys (unlike LineageOS for example)

you can just use any other OTP application on Linux like https://gitlab.gnome.org/World/Authenticator or https://apps.kde.org/keysmith/, they all follow the same protocol
you can export your keys in Aegis and import them in most applications