I find it bizarre that they felt the need to specify "Asian". What does that have to do with anything?
That Vietnamese surname of Nguyen is formally written as Nguyễn in the current script, or as 阮 in the old adapted Hanzi script, and practically pronounced somewhere around "gwen". Most other EA countries tend to do equivalents of "阮" -> "gwen", and East Asian cultures generally prefer Last-First ordering, which tend to put more information in the first name, and so there are going to be tons of last names in Latin that end up being barely unambiguous to be a form of identification.
These can be a problem if email addresses are going to be formulated as e.g. "阮 必成" -> "Nguyễn Tất Thành" -> "Gwen Tat Than" -> "t.gwen@example.net", even if the surname is going to be that much long(although Nguyen in particular is just identical even in Vietnamese).
(Japan may be less affected thanks to having default 2+ char random surnames farmers made up in modernization efforts, but there are still the 灘(nada) highscool, the 津(tsu) city, etc etc that can be NLP problems)
Japanese has several advantages:
1. Both given names and surnames often use more than 2 syllables. This makes it more familiar to speakers of European languages, where multi-syllable words are no big deal.
2. English speakers find Japanese easier to pronounce and remember due to its simpler syllable structure and use of globally more common sounds compared to the other languages.
3. Unlike the other languages, Japanese names are always given in Given Name, Surname order in English media, while names from the other languages are inconsistently transcribed in either order.
4. Japanese is the "default" East Asian culture for the West in many ways, since it's the one that at least the US has most deeply engaged for the longest time. Many, many cultural exports that actually originated in China were introduced to the West via Japan with Japanese names. Also, Japan has been an advanced industrial economy since the late 1800s, while China only achieved that status a few decades ago.
5. Japanese transcription into Roman characters has remained consistent for a long time (helped by the simpler phonology). Chinese and Korean have been cursed with several incompatible transcription systems over the years - although Pinyin has been winning out for Chinese, there are still many names with legacy transcriptions, not to mention the other Chinese languages needing their own transcriptions. Vietnamese does exclusively use Latin characters, but the diacritics and unfamiliar pronunciation makes them harder to remember.
Upward of 40% Vietnamese have surname "Nguyen".
"Li" is quite popular in China.
Specifying "Asian last name" in this context is quite apt.
There's probably a little bit of that, but also simple carelessness.