I shall refer to a higher authority:

> A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause.

That is from Modern Operating Systems, 4th ed. (2015), by A.S. Tanenbaum & H. Bos, page 439. (By "process" they really mean "process or thread".)

One way for thread A to wait on thread B is as you said: thread A can request a lock that thread B currently owns; then A waits for B. So requesting locks in opposite orders can result in deadlock. But of course if there are no locks, then no such waiting occurs, and so deadlock from this cause is impossible.

But there are other ways to wait. If var_a and var_b are shared variables, then code like the following might result in deadlock (The following code is in C; to simplify things, I'm assuming that data races and memory-access ordering are not issues.)

Thread A does:

    var_a = 1;
    while (var_b != 0) ;
    var_a = 0;

And thread B does:

    var_b = 1;
    while (var_a != 0) ;
    var_b = 0;

If Pony is capable of executing code like the above, then deadlock is possible.