It's not slavery if one is willing to be enslaved.
That is simply untrue. We sell our freedoms all the time. I sell big chunks of mine in two-week increments when I work; I sell my freedom for other times when I go home to an apartment where there are certain limits on my behavior.
That person may become a slave by choice, but only at the loss of freedom. Because the negative ramifications of slavery are not presented to the potential slave, they are making a choice with limited information.
Arguably it would be slavery if they agreed at first and then decided they wanted out, but the statement is not obviously bullshit.