From the bottom of the article under "Origin of the File":

  The source was posted on the Korean-language blog
  6502.tistory.com without further comment, in a marked-u
  format:...
  [ lots of interesting background here ]
  ...
  Given all this, it is safe to assume the file with the
  Microsoft BASIC for 6502 source originated at Apple, and
  was given to David Craig together with the other source
  be published.

So my guess is potentially safe to look at, but that's about it. If you have any doubts, I'd recommend not reading the original post.

It should be safe to look at from an archaeological perspective. You're not going to get any competitive advantage out of this or achieve anything by publishing a derived work unless you have access to a time machine.

You know, in case you are working on any competing BASIC for some hot new microcomputer coming into the market later this year.

Or you know, you decide to bundle it in an emulator for fun not profit, then get sued by Microsoft.

distributing ≠ reading

I was responding to johansch's new trail of thought. He is implying more than reading.

I didn't read it as implying more than reading. Reading alone could get you into trouble if you end up making something similar from scratch. AIUI/IANAL, there's not a legal requirement to implement competing products in a clean room fashion, but it's easier to defend against a lawsuit if your implementers never read the competing code.

Yes I would agree. There is reading as in opening the link and thinking what the hell, glad I don't have to code in assembler. (What I did).

Then there is reading as in opening the link, examining the code, deeply understanding it, then going off and writing your own one 'from scratch'.