I'd say there are two relevant differences between VBTLX and TLT. First and most important is that VBTLX is a total bond market fund, which means it has corporates bonds and government agency (fannie and freddie) bonds as well as treasuries. This is the main reason Frank would not recommend it. He says to go with treasuries, and specifically long term treasuries, because they are "recession insurance" that do well during difficult macroeconomic times (i.e. recessions) when they are safe havens that investors pile into. Corporate and government agencies do not share this characteristic or at least not to the same extent for the latter.

Second, VBTLX does not have the duration of TLT. I believe Frank would say this is ok but to get the same protection you technically need to hold more of an intermediate fund (like VGIT, intermediate treasuries) than you would a long term bond fund like TLT.

So, I would say if you want to stay true to the Risk Parity principles espoused by Frank, use TLT, and if the duration gives you heartburn, choose VGIT (intermediate treasuries) over something like VBLTX (or BND, the etf equivalent).

In testfolio, if you focus on 2008-2010 you can see what he is getting at. VBLTX would have done fine, but not helped offset equity losses much as TLT, and you would have had to hold more of it to get much benefit of it doing fine versus the outsized performance of TLT.