I can believe it is frustrating. But the second salesman was being frank (and kind). It is extremely likely that your request does not fit their business model. (The assembly folks I know of in that area would not pursue small lots like 400 units unless the value of each module was very high -- say $10K per module.) I can tell you that salesman for the larger fab houses are
extremely responsive when they are dealing with established businesses or with people who have a track record in the industry. It is tough on the newcomer, but business is like that.
As for "turnkey" -- these folks are not in the business of renting out their equipment. They don't rent out the "ovens" or the pick and place machines. They provide a service that often, if not always, includes purchasing the materials, inventory, assembly, and test. Some will go even farther, including both up-front design-for-hire, all the way to final assembly, test, and delivery. They are not set up for short runs. They are not set up to store and load your parts. They
do work with startups, but will want to understand where the business is going. They also want to know that the design they'll be getting is reasonably likely to work -- they don't make a lot of money on prototype volumes, multiple passes are not in their interest, and they want to see that the time they invest will lead to more business.
If you've got 400 modules, and you want to supply parts, and all that stuff, you probably should look at some of the prototype fab services or small-lot services. Seeed Studio offers pcb assembly. Take a look at
https://www.seeedstudio.com/prototype-pcb-assembly.html I haven't used the service, but I have bought modules that were assembled by them and they were pretty good. The website says all the right things. They may not accept "customer supplied parts" but that should not rule them out.
You should be prepared to pay for the service -- machine time is expensive and they've got bills to pay. On the other hand, they appear to know what they are doing, and that is often worth paying for.
Good luck.