A lot of physical keyboards on devices label the alternative-function key modifier with "ALT".
Whereas on a laptop's keyboard you may be more used to this key being labelled as "Fn" and unfortunately, this key behaves very differently to the alt modifier that you may have been expecting.
It's effects are often different between devices and because of different keyboard layouts, it would become impossible for us to support translating ALT to the more typical "PC" definition of alt. This would also be undesirable behaviour for a lot of other JuiceSSH users.