Did they? It's easy to find people claiming they made the statement, but I've been unable to find a quote that actually traces back to the WHO.
Are they likely to have made the statement? No. According to a CDC study, in the United States (a location not generally considered a hotbed of starvation), the average adult woman in the year 2000 consumed 1877 kcal/day, far below the 2000 kcal/day threshold the WHO supposedly uses.
The 2100 kcal/day in the report found by Geobits is a population average: averaging across everything from a newborn baby to an adult male performing heavy physical labor.
Is there a definition for starvation based solely on calories? Probably, considering the number of people who like defining things, but I doubt anyone would take it seriously. Virtually everyone (eg. the FAO) defines "starvation", "chronic hunger", or "undernourishment" as simply "insufficient energy intake to meet energy needs".