> It still has to be proven that this can be done by a startup.

I am confused. What, exactly, has to be proven? Startups have already done both things [1] [2].

(also, correction from my op: s/fission/fusion)

[1]: https://www.helionenergy.com

[2]: https://boomsupersonic.com

You can categorize fusion reactors and experimental / prototype aircraft similarly:

(1) "Our engines produce some thrust, and our wings produce some lift".

(2) "We managed to take off and land!"

(3) A working aircraft - seemingly performant, reliable, and economical enough that actual commercial airlines want to buy it.

(4) Like (3), but you've made a bunch of 'em, delivered 'em to airlines, and they're in regular real-word use...without said airlines suffering from buyer's remorse.

My understanding is that both Boom Supersonic, and everyone trying to invent a fusion reactor...are all still in category (1).

> Startups have already done both things

Startups intend to do both things. ;-)

I might be misunderstanding, but hasn't Helion already built prototype reactors, that do produce energy through fusion?

Not more energy than they put in. Just like every other fusion experiment.