If we ignore cloud interfacing solutions[1], there is a simple answer. "Enterprise scale" solutions really mean "Enterprise SCALING".
Pre-sales for 90 servers in many cases would be impossible. Pre-sales of a single rack mountable item that can then be expanded from depending on the workload is really attractive, however. Write the software right, and it should be able to cope.
Pre-sales probably starts with a customer renting a single box to try out, possibly with some professional services time. The customer decides they like it, and commission a project to develop a Watson solution. IBM pre-sales then work out how many servers would actually be needed, once the customer is interested. As the solution can scale, when management goes "that cost looks little high" they can safely say "well, if we reduce it by sixteen servers, you'll be slightly slower but that might be ok. Then if the business decides to increase we can simply add more servers[2] at a later date". And, of course, adding more servers at a later date means more professional services time, meaning more money and more quality customer relationship building time.
[1] Which for all kinds of commercial reasons, we have to discount a cloud solution being the default here. Buzzword, yes, but enterprise is slow and many really exciting uses cannot be based in the cloud.
[2] Or, y'know, just turn this key....