Actually I think what I said was completely accurate (as far as it went), but you are completely correct that it makes a difference how you are sending the email ... and the confusion seems to come from which "user" we are talking about as a "machine user".
My original answer was directed solely at "website" generated email that does not use SMTP (php mail(), sendmail, etc), where the limit is actually 200 per hour, and the quota is based entirely upon the machine user under which the domain is run.
Your response from support deals with "the other quota" of 100 emails an hour for each "machine user's" SMTP usage - and I glad to see DreamHost confirm that they will allow that quota for each "email account". A full "mailbox" email account is, in fact, a "machine user", albeit one with limited capabilities.
As you noted, where you are likely to run into issues is if you are hosting multiple domain run under the same machine user and using each of those website is sending mail from the website using non-smtp methods (which is why I focused on that in my response).
Thanks for the "heads up'!