iMac has weird startup chime and refuses to boot
No, you cannot assume that. This repeating startup chime with the three second pause between the three tones is an indicator that you are trying to boot your iMac from a non-supported operating system.

The 27" mid-2011 iMacs originally shipped with Mac OS X 10.6.6, and later shipped with Mac OS X 10.6.7. This iMac will not run from Mac OS X 10.6.5 or earlier. However, on the positive side, it will run all version since, including macOS Sierra.

As an aside, it is strange that Apple hasn't documented this on the page you referenced.

Hope this helps!