They're not tally marks, it's just a different way of encoding a positional number system representation. You would get the same effect if instead of writing zeroes you left empty spaces. 2_4 + 1_3_ = 1234. If you were writing in columns you would not need the additional glyph.
(creator here) +1, the different digits add trivially as you say.
I also thought it was fun that when you overlay digits, you do get 1+4=5, 1+6=7, 1+8=9, 2+6=8, and 2+7=9 . That one I only found accidentally after a bit of playing, so the demo note is more of a fun side note than a really useful property.
