I'm not sure this is really worth arguing over.
My point is that the starting point assumptions for humanism are ones that are almost universally inclusive, while the starting point assumptions for a theistic moral philosophy are significantly exclusive.
In both cases as you delve deeper into the layering of belief and morality therein there will be points at which people will say 'I'm out' (in a Dragon's Den kind of way) to either humanism or theistic moral philosophies. The difference being that for humanism pretty well everyone is in at the start (as the basic assumptions are pretty well universal and non controversial). For a theistic moral philosophies many people have already said 'I'm out' at the very first assumption - that there is a god.
This is a very good point - unless you have a card-carrying IS member on board then pretty much everyone, theist and atheist alike, can start a dialogue from the same few broad tenets of humanism, whereas if you start with two lots of theists of different brands, or start arguing from a theistic p.o.v. with an atheist, you'll knit sawdust before you reach a consensus.
Richard Holloway, recently mentioned on here, wrote an entire book called
Godless Morality about keeping religious assertions and assumptions out of civil discourse on things such as abortion, euthanasia, marriage equality and so forth. It'll never happen of course and he may be thought foolhardy even to float the idea, but he was to me self-evidently right.