Hashicorp is doing a smart but normal thing for on-prem sizing there - they are recommending specs which ensure you have no problems for a lot of workload sizes. And "workload" can grow very large there, since a single DC can handle up to 5k clients and such.
IIRC both k8s and nomad rely on the Raft algorithm for consensus, and so both of them inherit the requirement of a minimum of 3 nodes from that algorithm.
If you want something to run some containers in the cloud, and you aren't concerned by the occasional failure, then there is no issue running a single Nomad (or k8s) on a single machine and later adding more if and when you require it.
And yes, nomad recommends 3 - 9 nodes in prod, maybe 11 or 13. The 3 are necessary to be able to tolerate one broken/maintained node and maintain the ability to schedule workloads.
You can increase the number of tolerated nodes by increasing the number of nodes - 5 tolerate 2, 9 tolerate 3, 11 tolerate 5, 13 tolerate 6, but raft becomes slower with more nodes, because raft is a synchronous protocol.
However, it is entirely possible to run a single node nomad system, if the downtime in scheduling is fine.
We run a Raft cluster of 5 voting members with 13,000 physical Nomad nodes mixed across OS's with a bunch of random workloads using docker, exec, raw_exec, java, and some in-house drivers. I'll clarify that (and I can't... because I can't edit my post anymore :( ).
We're nowhere near that size, but the architecture is probably identical.