"Do not use present continuous with for or since" is a simplified guideline rather than a rule. The core of the issue here is that whatever time reference you are using must be in agreement with your verbal tense. It just so happens that most usages of for or since provide a time reference that doesn't work with the present continuous, but it isn't impossible as in:
We are waiting for the time being
And colloquially you'll even see things that deviate a bit from standard grammar rules, such as:
We are waiting since noon
If we were being more strict about grammar we could use a number of different tenses depending on the exact context or intention.
Past continuous implies the waiting is now finished:
We were waiting since noon
The present perfect could suggest the waiting has only recently finished or even that it is still ongoing:
We have waited since noon
The present perfect continuous suggests the action is still ongoing and emphasizes the length of it:
We have been waiting since noon
To recap, what matters is for our time reference to be in agreement with the verbal tense used. However, in colloquial situations it is reasonably common to use the present continuous with time references that link to a past moment.