He's right. All you have to do is roll and form plate. Any items that cannot be done in house can be subcontracted locally then brought back to the workshop for assembly.Locomotive boilers are low pressure toys compared to what companies like this normally turn out. Modern industrial boilers are quite different beasts. Locomotives are low pressure fire tube boilers, I believe most industrial boilers are high pressure water tube jobs.
Then hire competent welders to weld the thing together.
Companies such as L&A Pressure & ER Curtian in Sydney turn out all sorts of pressure vessels and petrochemical columns and reactors all the time.
While I'm sure their experience would have enabled them to build a locomotive boiler with little trouble at all, but you can bet the railway request for tenders probably stipulated 'experience with locomotive fire tube boilers', and the locals, while perfectly competent boiler makers would find it difficult to claim low pressure fire tube experience since probably all their work would have been on water tube types.
The locals were probably all knocked out on that alone.