That page lists it as a two syllable phrase (look at the categories at the bottom).
English speakers would be tempted to think it as a three syllable phrase because ʁjɛ̃ isn't a possible syllable in English, but in French "de rien" should be two syllables.
This is similar to, e.g., English speakers incorrectly pronouncing the Spanish word "luego" with three syllables.
Random fun fact: when an English speaker says "strike", a Korean speaker hears five syllables.
seu-teu-ra-i-keu
(Well, to be fair, it's more like "an awful lot of sounds that have no business being in the same syllable merged together", and less "five distinct syllables", but still that's how Koreans end up writing the sound in Korean.)
I should have specifically called out the audible pronunciation, where it's clearly more than two syllables in pronunciation. As pointed out below, it has "two and a half" syllables, I guess the site has no category for that.
No, you're misperceiving "rien" as having two syllables because you are (I'm guessing) a native English speaker. You can't just "hear" how many syllables something has as syllabification rules are language-specific. The nearest corresponding sequence of English phonemes would have to be split into two syllables - but English has different syllabification rules.
Native French speaker here. Maybe it's confusing to discuss this in terms of syllables unless you are a linguist. If you just want to learn how to say the word, what matters is how people actually say the word. Specifically, you pronounce "rien" in one quick shot. (So it's definitely closer to one syllable if we care about this. But if I am to say the word very slowly to teach somebody how it sounds, it might progressively become closer to two syllables. Languages are complicated.)
>But if I am to say the word very slowly to teach somebody how it sounds, it might progressively become closer to two syllables. Languages are complicated.
I'm sure that's true, but I don't think it gives rise to any real doubt as to the syllable count. For example, you could stretch out the English word "near" into two syllables in very slow and deliberate speech, but it's definitely a monosyllabic word in most (possibly all?) varieties of English.
For "rien" in particular, I'd say speakers can do that diaeresis as a sort of emphasis. For instance "Tu n'as rien fait! Rien! ("you did nothing! Nothing!"). The first "rien" can be said as a single-syllable world, while the second one can be pronounced as a two-syllables.
It also may be pronounced slightly differently in various places in France (regional accents).
English speakers would be tempted to think it as a three syllable phrase because ʁjɛ̃ isn't a possible syllable in English, but in French "de rien" should be two syllables.
This is similar to, e.g., English speakers incorrectly pronouncing the Spanish word "luego" with three syllables.