This is not correct. All of the ARTCC frequencies are indeed in the database and downloadable via software built to access the database. The difference is that they were the original Nationwide frequencies and are in a class all by themselves. They are searchable but you must use "-Entire Database-" as the state you select to get the response you are looking for. This is not without a bug however, in that only the first entry in the ARTCC section will be returned. So searching for the 134.125 on the entire DB returns the entry in ZOB (PA) ARTCC section. I think I remember this being submitted as a bug previously but I will check later and if I can't find the report, I will resubmit it.
I don't want to get in a back and forth about this, but for the purposes of the OP's question which related specifically to Colorado, and why he wasn't finding those frequencies in a search of Colorado, everything I said was 100% correct.
As it pertains to the Denver (ZDV) page, all of the frequencies listed at the top under
FAA Reported Information for ZDV are in an HTML table, they are
not entered in the DB under ZDV.
If you go to the Cleveland (ZOB) page, you will find that 134.125 is listed at the top of the page in a table like ZDV, but is
also entered in the DB under
Warren, PA - RCAG, which is why it returns in DB wide searches.
I just verified with 4 different software packages, and none of them were able to access 134.125 from ZDV, as I expected.