From a review of the Coleco Adam by Jules H. Gilder in the April 1984 issue of BYTE (nearly half the article is dedicated to discussing SmartBASIC) we can glean some answers. The compatibility with Applesoft BASIC is clearly an intentional feature, but it's also clear that it's not a Microsoft product and differs in some significant ways.
Although Coleco boasts that its SmartBASIC is compatible with
Applesoft, there are a number of differences in the languages.
SmartBASIC was not written by Microsoft, as was Applesoft, and does
not have the same internal construction as Microsoft BASIC.
We can also see from this article that various more modern features and bug fixes were included that are not present in Microsoft BASIC.
A positive result of this difference is that new ideas in interpreter design were
included. For example, SmartBASIC checks syntax on entry. SmartBASIC
is also more highly table-driven than Microsoft BASIC, increasing the
operating speed. [...] Another advantage of SmartBASIC not being
written by Microsoft is that it doesn't have the bugs associated with
Microsoft BASIC. The author of this language made sure that all
floating-point numbers are properly represented, with no round-off
errors occurring as with Microsoft BASIC.
Here we can also infer that it's not a product of direct reverse engineering due to the structural differences mentioned.
The SmartBASIC interpreter's unusual way of storing program lines in
memory was less understandable to me. Program lines don't have the
familiar Microsoft line structure (2 bytes point to the next line, 2
bytes for the line number, a tokenized line, and a 0 as a line
terminator). In SmartBASIC, the line numbers and next-line pointers
are stored in separate tables elsewhere in memory.
The review also describes some key differences including here where it shows that SmartBASIC hews closer to more modern implementations of Microsoft BASIC than Applesoft BASIC does.
Another annoying feature of SmartBASIC is that it requires spaces
between keywords, as do later versions of Microsoft BASIC. Applesoft
is very tolerant of this sort of thing and is smart enough to
recognize most keywords without spaces.
Given all the above it seems clear that no Microsoft code was used and we can conclude that the Adam's SmartBASIC is an original creation made with Applesoft BASIC compatibility in mind.
