If you open the substeps of step 22, you'll see the zipping and uploading sub-steps that do seem to do something sensible: 
https://luci-logdog.appspot.com/v/?s=chrome%2Fbb%2Finternal.bling.main%2Fcanary-simulator%2F1213%2F%2B%2Frecipes%2Fsteps%2Fupload_Chromium.app%2F0%2Fsteps%2Ftar_Chromium.tar.gz%2F0%2Fstdout
https://luci-logdog.appspot.com/v/?s=chrome%2Fbb%2Finternal.bling.main%2Fcanary-simulator%2F1213%2F%2B%2Frecipes%2Fsteps%2Fupload_Chromium.app%2F0%2Fsteps%2Fgsutil_upload_Chromium.tar.gz%2F0%2Fstdout

and the build in question uploaded Chromium.tar.gz to https://pantheon.corp.google.com/storage/browser/bling-archive/canary-simulator/1213/

Is this WAI or do you expect something else that I'm missing?

Your pantheon link is from Dec 2016. I can't find recent examples of this working recently.

If I go to https://uberchromegw.corp.google.com/i/internal.bling.main/builders/canary-simulator/builds/1223 which is from Oct 2.

Click on Step 22 cache, I see a lot of errors, which I'm not sure if that's impacting upload. 

Now if I go to pantheon and I search in pantheon for prefix 1223 to see if this sim build was uploaded, it says this is also built in Dec 2016 - obviously not the one from today.

Actually, Chromium.tar.gz is from 9/20/2017 in https://pantheon.corp.google.com/storage/browser/bling-archive/canary-simulator/1213/ .

The confusion here is because we changed the builder name twice, and this reset its numbering. Therefore, it uploaded the zip over anther old build that indeed was from Dec 2016. Only these days we upload a single zip file rather than all the files separately, so it _looks_ like all the files are old :-)

So much for relying on Buildbot build numbering... Tangent: we need a new scheme for upload paths, since Buildbot is going away, for real, and reasonably soon.

#3, tangent-related question: while buildbot is going away, aren't the concepts of builders and build numbers are staying for the foreseeable future in LUCI land?

Thanks for the clarifications on comment #4 Sergey. I see that only the chromium.app there is indeed from 2017 now. 

Yes, can we please change the numbering being used as this is not working well at all. Do I need to file a seperate bug for that or is there already one filed?

Re: #c4: IIUC, LUCI is trying hard to deprecate sequential build numbering, and replace it with a random globally unique build ID (currently, swarming task ID).

Re: #c5: sorry, I can't easily change the numbering, because, uh, Buildbot... I'd go for changing the upload path so it doesn't have a build number. We can use e.g. a date or a timestamp for a sortable unique ID instead, this will survive the LUCI transition.
Filed issue 771276.

The blocking bug should've effectively fixed the issue. New artifacts will now go into a subfolder named after the current timestamp, e.g. 20171010165300 (that's year-month-day-hour-minute-second lumped into a number).

Marking as fixed, please reopen or file more bugs if there are still related issues.

The first successful upload with the new scheme: https://pantheon.corp.google.com/storage/browser/bling-archive/canary-simulator/20171011013301/