Please don't close bug reports before there is some kind of consensus or some days for a "three-way handshake".
The 4.1.0 PDF chapter 10 contains "19 for 64-bit systems", I think that's wrong. Otherwise section 10.5 would say "If the default DIGITS value is 18, the whole number limit is 999999999999999999", and 18 or 19 nines are more than the 15 digits in my example. I guess (= I'm not sure, because it is apparently not mentioned in the release notes, or I missed it) that the 18 or 19 for 64bits platforms was at least partially dropped.
If that guess is correct the 15 digits in my example are more than 9 and as you said invalid. Section 10.5 in the PDF matches "Numbers used directly by REXX" in TRL section 11, the error code 26 is also fine. But ooREXX offers additional detailed error texts, and it does not need to say that a whole number is not a whole number, when the real problem is "abs(x) is greater than the implementation limit 999999999". I suggest to improve the appendix C.1.23 string for error 26.008 and similar 26.nnn strings to avoid the "high astonishment factor" of these error messages when x actually is a whole number.