I would strongly suggest you read through the application materials yourself and double-check so that you can push back and ask questions if you doubt the ability of your consultants to respond well. Have you already submitted your application? If not, you can revoke at any time and prepare and submit yourself - but obviously you may lose what you've paid up-front.
At any rate, look at page 5 of the IMM1344 form, which is the part that asks for your notice of assessment. It says quite clearly if you cannot provide the NOA, you must provide an explanation on a separate sheet of paper, accompanied by relevant financial documentation (i.e. to show you can support, this might be bank account savings, other info on income, etc.)
And your consultant is not entirely correct that it is not possible to "link" information later on. You CAN submit a webform after the submission to provide the NOA when available. It is more difficult to submit this information - because you may have difficulty specifying sufficiently the file without the file number, and IRCC may take some time to figure it out, as it won't be high priority - but it can be done.
Now it's up to you and the consultant whether it is better to wait for the notice of assessment or if what you have already in hand to show ability to support is sufficient. That's a separate question, and it may be better to wait until you get the NOA (I've no idea). But it is certainly possible to submit without waiting and to link information later (although not necessarily easy).