I'm not sure we can apply legislative history beyond what was actually written by the legislature.  Congress certainly has the authority and ability to write tax law that specifies how 529 plan assets and distributions should be considered for the purposes of determining support.  They did not do so.  What they did was to carve out how contributions and assets are to be treated FOR GIFT TAX PURPOSES in Chapter 12 of the IRC (and GST purposes in Chapter 13), as codified in 529(c)(2).  Again, they had the opportunity to specify how the law operates for other purposes (such as section 152) but they did not do so.  The IRS has the ability to issue regulations in this area, the closest we might have are some ideas regarding gift taxation issues presented in an Advance Notice of Proposed Rulemaking (that do not specifically address this situation but do help to clear up some other open items):

https://www.govinfo.gov/content/pkg/FR-2008-01-18/pdf/E8-859.pdf

To the best of my knowledge, the proposed rulemaking never actually happened, we're just left with "here's what someone at the IRS was thinking about back in 2008."

To be clear, I'm not arguing for one side or the other, my argument is simply that we don't know.  Personally I think there exists a reasonable basis for whichever position you want to take here so I would opt for the position that benefits the taxpayer the most.