No, a single check mark means the message has been delivered but not yet opened. It’s possible that the person either didn’t have the time to open your message but went online to check on some important conversation or just decided to leave your conversation for later. If the check marks change to double - that’s when they’ve read the message.
It’s also possible to preview a message if you’re using Telegram X for Android by long-tapping on a chat. This will bring up a preview of the chat and you can quickly look at what’s been sent. So maybe the person checked out your message but didn’t open the chat.