The problem with the Truss mini-budget was not that they had an ‘unfunded’ deficit but that there was no mechanism between the deficit and investment. Rich people tend to save and for the deficit to do any social good, the banks would have to lend to investors i.e. entities looking to buy or make capital goods; which they have never done. Private sector domestic investment has usually been funded by retained earnings!
The lesson here is that the markets were not frightened of the deficit, just its purpose.
See also Growth vs public debt management on this blog.
More on growth & debt