Commercially viable means more than getting energy out of it though - it also requires that the build, operation and maintenance costs over the lifetime of the machine don't outweigh the value of the net energy generated. Of course it needs to work on paper before you build it, but this is experimental science and until you have built it you haven't proved it.
Customer "partners" who are looking ahead to future supply needs, are often willing to help cover initial capital expenditures of promising suppliers, to get them to positive operational cash flow faster.
Then become a regular (but favored) customer, as the supply side becomes truly net profitable.
Yes, but in this case it seems there is still some(!) science to be verified and/or worked out, not just a matter of money and working out operational efficiencies.
Even once they demonstrate fusion and viable levels of power extraction, it seems there would still be an issue of what is the level of "wear and tear" (incl. any radioactivity) of the fusion chamber - which it would seem may ultimately be a matter of try it for an extended period of time and see.
That's what a good/motivated early customer partner is for. Sharing the risk, benefiting from the new supply if things succeed.