The lesson says: Notice how, sometimes despite using the singular "lo + adjective" (lo menos caro), we conjugate the verb in the plural form (son). This is because, being a neutral idea, it can refer to a group of things, i.e "The least expensive things..."
This wording is not clear to me. Do you mean to say that the verb is single or plural depending on the object of the sentence? For example:
Lo más impresionante del viaje fue la cantidad de museos interesantes.
"la cantidad" is single, so "fue" (single) is used - is that correct?
Lo menos caro de esta tienda son los accesorios.
And here, "los accesorios" is plural, hence "son" (plural) - correct?
