I want to make a record that establishes that certain content existed on Internet at some point of time. It seems that
is a way to archive the page. But, are there any other methods of proving this?
It depends on what jurisdiction you are located in. In some US states, a notary public can prepare a certified copy of most written documents. The certification would include the date the certification was made. However, it would be a copy, so the copyright terms for the item would have to allow the making of a copy.
If you want a record of what the page looks like right now, archive it using perma.cc or archive.is, or just print it off and have it notarized.
If you want a record of what the page looked like at some point in the past, try www.archive.org and check the closest dates before and after the date you're targeting.