Literally Cambridge University in 2004 and recently again

You will often hear less used with plural countable nouns in informal spoken situations, but traditionally it is not considered to be correct:

We’ve got less pizzas than we need. There’s ten people and only eight pizzas. (traditionally correct usage: fewer pizzas)

And a good blog post on the topic that actually fits to this situation

He goes on to point out what less/fewer purists often ignore: that even countables may, in a given context, be considered as quantities rather than numbers. "For instance: 'Not many of these buildings are fewer than thirty years old.’ The thought here is not of individual years but of a period of time; therefore, less."

Which is precisely what's happening here.