What's the risk Borja will run into someone who will want his cash ... but at the same time that person doesn't have plans to exchange bitcoins to get it from him?
Well, there were two of them traveling together. That makes things a little safer.
Also, if you read the OP:
we are in an urgent need of exchangers, who would like to exchange bitcoins to cash in each country.
So they were only selling bitcoins, in exchange for cash. And since nearly each country had its own fiat, there was no point in holding lots of cash -- and thus risk was muted a little from that as well.
If I remember correctly, they had more concerns about their cash going to "checkpoint guards" than to thieves [edit: who don't work for the government] .