Ken,
They take the moniker from a parable and apply it literally to themselves. Why would anyone would think Jesus would view his brothers and joint heirs as actual lowly slaves?
Paul calls himself a slave of Christ and God to reinforce the extreme efforts to which he went in spreading the gospel. He calls attention to some few as fellow slaves of his, but these also suffered for the sake of the good news. By and large, Christians didn't regard themselves as slaves. Paul made an excellent argument against Christians viewing themselves as slaves.
Galatians 5:1 — For such freedom Christ set us free. Therefore stand fast, and do not let yourselves be confined again in a yoke of slavery.
I agree. It is an example of false humility for them to continually paint themselves as slaves while also claiming to be already appointed over all the master's belongings. Someone over all the master's belongings isn't a slave anymore. They have all authority in the house.
Respectfully,
AuldSoul