Starting at 1st level, your alien knowledge gives you the ability to touch the minds of other creatures. You can communicate telepathically with any creature you can see within 30 feet of you.
You left out a part that from my point of view gives at least a hint what is intended. The paragraph continues to read:
You don't need to share a language with the creature for it to understand your telepathic utterances, but the creature must be able understand at least one language.
That is the full text. Remarkably, nothing is said about the warlocks ability to understand the reply. The focus is solely on the receiver of the warlocks message. If the communication was meant to be both ways, I would have expected this to read:
You don't need to share a language with the creature to communicate with it, but the creature must be able to communicate in at least one language.
In contrast to the wording of the ability in question, the spell telepathy for example is very clear about the communication being both ways:
[...] you and the target can share [...] with one another through the link.
As this ability is worded specifically for sending messages, I suspect that it's actually intended to do just that. It lets you send messages. One-way.