Not to go into much detail, but these author's views are not shared broadly in the cosmology community. There was a recent paper [1] put on the arxiv that talked a bit about this. I'm not an expert on supernova cosmology (I work on cosmology using other methods) but people I know who are experts claim there are issues.

There are also a ton of other reasons to think that the universe is flat (see the CMB [2]) and to think that there is dark energy (large scale structure [3]).

We still have no idea what DE is and it might not be an "Energy" but rather some modification of gravity or something else. But we're pretty confident in the phenomenology - there is something there that appears to be pulling things apart.

[1] https://arxiv.org/pdf/1911.06456.pdf [2] https://physics.stackexchange.com/questions/439535/why-does-... [3] https://www.darkenergysurvey.org/supporting-science/large-sc...