At this point, Arbortext Editor presents a Textbox for XSD Attributes of type xs:boolean. An xs:boolean is constrained to the values "true", "false", "1" and "0", which must be written in exactly that manner (a simple case deviation like "True" or "TRUE" is rejected with an error message).
Since the boolean can actually only take 2 possible values (with "1" being synonymous to "true" and "0" being synonymous to "false") it would make sense to present it as such.
Probably the more understandable option would be to use a simple Check box that presents "true" and "1" as being checked, while "false" and "0" are shown unchecked.
The easier option on the other hand might be a Selection list (similar to enumerated values), but that leaves the question of which values to present (true/false vs. 0/1).
Since both options need a way for the Doctype (and/or Stylesheet) author to specify whether true/false or 0/1 should be used, an according DCF Option would probably be needed aswell.
Possibly related: the Styler Graphic attribute "scale to fit" seems to only accept "1" (at least with 6.1 M010) to re-scale an illustration; while "true" seems to be ignored and presented as if it were "0".