He effectively gave us the equation and kept secret most of the coefficients (though he explained how to choose many of those coefficients). That wouldn't be acceptable for academic purposes, but it seems quite reasonable for journalistic purposes (and is certainly more than any typical political "analyst" provides). He also couldn't give out all the raw data that went into the analyses, because much of it was stuff he had to pay for. Getting past those paywalls and assembling a comprehensive database of polling information is almost certainly the bulk of the work necessary for reproducing his results.