"His webpage is in Esperanto."
Translation:Lia retpaĝo estas en Esperanto.
Webpage is retpaĝo. A Website is a bundle of webpages, that is retpaĝaro. Retejo is a network. The word is not necessarily related to a webpage or website, but modern networks often have got websites.
"Retejo" is often used to mean "website". Take a look at Reta Vortaro and Komputeko. In context, it could mean the "site of a network", but if you just want to say "network", that would be "reto".
website = retpaĝaro, retejo
webpage = retpaĝo
network = reto.
Technically a 'retejo' is a bit more general than a 'retpaĝaro'.
Things like FTP servers and folders, IRC nodes and channels, BBS, software repositories, etc... this locations are not website, but are places in internet.
Shouldn't "en" only be used when something is literally in something else?
This is using "en" as if it were the English word "in" which is easy for everyone here who speaks English but not consistent and understandable for Esperantistoj who don't speak English.
I feel like "je" should be used here or perhaps this sentence should be translated completely differently, ie "Tiu retpaĝo skribata esperante."
I think that would mean that it's a retpaĝo about Esperanto or something Esperanto related. Rather than it just being written in Esperanto.
What's the difference between retpaĝo and retejo? Webpage vs. website? Do these actually mean different things?
A website may contain many webpages. See also: Google, what is the difference between a website and a web page
Aren't languages usually referred to with the phrase "the adverbial language"? E.g. " la germana", "la franca", etc. Shouldn't the same be said of " la Esperanta"?
No, it should’t, languages that are not connect to a people or country are nouns: Esperanto, Volapuko, Ido, Tokipono, Tokpisino etc. in contrast to la franca, la angla, la sveda, la klingona etc.
Laŭ PMEG: O-vortajn lingvonomojn oni principe ankaŭ povas uzi en A-vorta formo. Tiam oni ja uzu la: la Esperanta (lingvo), la Sanskrita (lingvo), la Urdua (lingvo), la Ida (lingvo). Sed normale oni uzu la simplan O-formon sen la.
So, the A-word form is acceptable, just not normally used.