GeoTiff Codes out of Range

According to "http://geotiff.maptools.org/spec/geotiff6.html" as well as the "GeoTIFF Format Specification" the GeoTiff-Codes are expected to be in between Ranges for the different Sections. For Example Projected CS Type Codes should be in between 20000 and 32760, but in the "https://github.com/ufz/geotiff/blob/master/csv/alias.csv" there are Codes like 66156405 which are even outside the Private User Implementations - Ranges. Shouldn't it be avoided to use Codes that are not Specification-conform?