Look up this: https://en.oxforddictionaries.com/definition/most Adverb meaning 2. It's an informal usage but is also definitely natural. The writer you quoted may have thought too much about the common usage of "the most", then concluded (wrongly) that we always need "the", and that not saying it is lazy. There is no difference between US and UK English usage on this issue.

There's nothing unnatural about 'I like you most' Who told you this? You were told wrong information.

Thank you for your question. Perhaps, you should take a lesson with a native speaker of English on Italki? Then, they could explain these forms more clearly to you. Who is the one I love the most (of all)? I love you the most (of all). Truncated by natives as 'I love you most'. But usually, I would say "I love you the most'. Who do I like the most (of all)? I like you the most (of all). We are best friends. I like you most. Again, usually, I'd say 'I like you the most ('of all' out of the bunch). It's not necessary to understand grammar to such a high-level to speak English correctly.