That's to say, the message sent via non-default SMS App will be split evenly automatically by Android System with version 4.4 + (including 5.0+), if it exceeds the 140 character (approximate number, carrier/country dependent).
And AirDroid SMS feature, as you might understand, can still not be defaulted as SMS App.
For more information about this issue, you can refer to this explanation by Google: https://developer.android.com/about/versions/kitkat.html#44-sms-provider
We would love to make any improvement possible, whenever it's technically feasible.
Note: A character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language.