The converse actually does appear to be true. Be careful with the coding of the unharmonized source variables in the 1980 sample, as most of them actually have a value of ’ 9999999’ for the ‘unknown/missing’ value (please refer to the Codes page for the relevant unharmonized variables listed in this previous answer). With that in mind, all respondents with a value of “unknown/missing” for INCTOT do indeed have a value of “unknown/missing” for all unharmonized source variables.
INCTOT does not have any values of “unknown/missing” for the years 2000 and 2010, because the unharmonized source variables do not have values of “unknown/missing.” While that is likely not the most satisfying answer, there does not appear to be any indication of “unknown/missing” income values in the source data provided by the Brazil Census. Thus, while it is possible that some respondents did not report their income in 2000 and 2010, there is no way to detect this with the available data. Note, however, that less than 1% of respondents in the Universe actually had a value of “unknown/missing” for INCTOT in the 1980 and 1991 surveys.
Finally, you appear to be correct about the reference year in the source document. We will correct this.
Hope this helps.