This is form
https://www.canada.ca/en/immigratio...-after-january-1-2016-completeness-check.html
You can argue to eternity and back, but that does not change what IRCC has stated. Anecdotal evidence does not become law unfortunately. There are also more number of applicants who have successfully submitted their applications without a copy of ECA and IELTS.
Also, in the GCMS notes it clearly states that the ECA and language scores verified on the issuing agency. If these documents were mandatory, then there would have been an upload link in the document submission.
Some applicants incorrectly enter the ECA report and or IELTS report number, and if the numbers are incorrectly entered, IRCC cannot verify them on the issuing agency website and this leads to rejection. This is why it is advised submit the documents with your application.
There are many applications rejected due to a PCC missing, even when it was uploaded, or the expiry date on the IELTS etc. This is where you file for a reconsideration.
IF you want to submit a copy of the ECA and IELTS with your application, that is your choice and your prerogative. However, the OP's questions was they missed filing them with his application and will there be a concerns.
The answer above from an IRCC source and not created by me.
If this is totally untrue, then please provide an IRCC link or point to the IRPA where it states that for the purpose of completeness check IELTS report and ECA are "
MANDATORY." Anecdotal evidence unfortunately does not make something mandatory.