The main reason secret chats are opt-in is that there is no key management built in so secret chats are not synchronized across devices. Also secret chats are not supported in the desktop yet.
That may or may not be true, but unfortunately many people think Telegram uses end-to-end encryption by default. This is why I keep posting the link to the FAQ ;).
It's just a Telegram bot, it's not "based on" Telegram's source code (which is what I assumed when reading your comment before opening the page). That's like saying Facebook is based on PHP and saying "that's a major red flag!"
Yes? That would be a major red flag if it was being touted as a feature. Especially if you add in a claim of "the most secure and fast programming language".
That's indeed a very dubious claim, only the opt-in "Secret Chats" use end-to-end encryption. [0]
[0] https://telegram.org/faq#q-so-how-do-you-encrypt-data