Let's say I'm writing a trivial proxy which simply forwards data unchanged between two sockets. This is my pseudocode:
while true:
x = read s1
write s2, x
y = read s2
write s1, y
This can deadlock. If for example s1 stops sending data, read(s1) blocks, so the proxy won't forward any data from s2 back to s1, potentially leading to a full pipeline deadlock.
Now, the above can be expressed in purely non blocking fashion as follow :
letrec f = async_read s1, \x ->
async_write s2, x \->
async_read s2, \y ->
async_write s1, y, f
It still deadlocks exactly as the previous program.
I'm not going to learn pony for the sake of an example, but I'm 97.5% sure you can express the above in Pony, except you do not pass the continuation directly to read and writes but to the returned promise. Sure, the pony runtime will still work and technically the actor could potentially handle other messages, but that's not very useful for the end user that has to deal with a deadlocked pipeline.
So claiming that pony is deadlock-free might be true in a trivial sense, but in practice it is at the very least misleading and potentially dangerous.
edit: it is of course possible to write the program in such a way that it doesn't deadlock, the point is that if pony was truly deadlock free it would reject all programs that can deadlock.
edit2: and of course, if one were to show that the above program is in fact not expressible in pony, it would be a great result and I would want to know about!