I listened to it twice and she very clearly said, "de citron", not "des".
Most final consonants are silent unless the next word begins with a vowel. For example, in "des chiennes, " the S's in des and chiennes are silent. In "des hommes," the s in des is pronounced with hommes, which makes the phrase be pronounced more like "de somme"
All of that is true yet "de" has a different vowel sound than "des". That is how you tell the singular from the plural. As the final "s", as you say, is silent.
De does have a different vowel sound than des, true, but de is only used to denote possession, origin, cause, the way something is done, or to describe a noun with another noun.