EDIT It might be worth having a look at telegram today, they have announced some new features, including the unsed/delete anything.
Details are here: Unsend Anything
If you are in a channel, then the behaviour you have highlighted would be expected, you would be able to reach out to the creator and ask them to delete the message(s) or discuss being given access rights on the channel.
As per Telegram FAQ: The creator of a channel can broadcast messages, delete any messages, add members (the first 200 only), remove members, change the channel's name, profile image and link, as well as delete the channel completely. The creator can also add and remove administrators to help manage the channel.
Have a look at Telegram's website here for more information: What can administrators do?