Here's the 3 questions I'd ask myself: 1. can they make it? 2. can they sell it? 3. if they make it and they sell it, could others copycat them, then out-execute and take the prize away from them?
The easiest to answer is 3. Out of all the unicorn startups of the last 2 decades, this one is the by far the one with the highest barriers of entry for the competition. The only ones coming close are SpaceX and Tesla. Why? Let's say Boom makes the first sale in 2025. If anyone wants to enter the market, they won't have a product for at least 10 years, and Boom can already be at jet 2.0 by that time, well into the R&D cycle for 3.0.
The first question: can they make it? Concorde was introduced in "prod" 45 years ago, but had the first flight 7 years prior. Technology has made huge advances in half a century. Obviously computers in general, and computational fluid dynamics in particular are many, many orders of magnitude above what was available in the mid-60's. But other things experienced impressive progress as well: theoretical CFD, materials, manufacturing to name just a few. Jet engines are also much more efficient, and what's super-important: Boom need to just buy them form some suppliers, they don't need to invent them as was the case with Concorde. There are basically only 3 suppliers out there (General Electric, Pratt&Whitney and Rolls Royce), but their competition is fierce, which means the engines are both extreme quality and reasonable price.
Finally, if they make it, will they sell it? Yes, the market for business jets can't wait for a supersonic jet, no matter what price level. A Gulfstream G700 comes at $75MM, and it goes without saying that it is subsonic. Think of all those billionaires who spend hundreds of millions on luxury yachts. A supersonic private jet would be the most exalted form of conspicuous consumption. Do you gather Abramovich will sign up for one?
Concorde could not target this market segment, since it was made with government money. But a VC-backed startup most surely can.