Is this example equivalent to "skip on not zero accumulator"?
It's the same idea as skips on the PDP-8, as explained in this Q&A: Without the I bit, you "or" the conditions, and skip if at least one of the conditions is true. With the I bit, you negate the final result, which (using de Morgan's rule) is equivalent to an "and" of the conditions each negated by themselves.
So we get
640100 SZA skip on AC zero
640400 SMA skip on sign AC = 1 (negative)
640500 skip on AC zero or AC negative
650500 skip on not (AC zero or AC negative)
= skip on AC not zero and AC not negative
= skip on AC positive and not zero
And as described in the explanation of the Spacewar code, the last example actually gets the "unofficial" mnemonic of
It's unofficial in the sense that you need to define these mnemonics in your assembler source code:
and they are also not listed in the manual. But as the Spacewar commentary says, it was an encouraged practice.