The relationship is that they are taking an underutilized resource (trucks) and increasing utilization through their network much like Uber is filling cars.

The current "our business model" section in the letter is confusing and I think the letter should be more up front about what their value proposition is. I had to read their "nosotros" page to figure it out:

  Más del 25% de los camiones en Brasil, México, Colombia y
  Argentina viajan vacíos cada semana, dejando de lado
  ganancias potenciales de US$4.5 billones cada mes. Nuestra
  tecnología pone en contacto a quien tiene la carga con
  quien la puede transportar, así el transportador nunca
  viajan vacío,...

More than 25% of trucks in Brazil, Mexico, Colombia and Argentina travel empty every week leaving behind potential profits of $4.5B dollars. Our technology puts people with cargo in contact with transporters, so the transporter never travels empty,...

It appears they are filling empty trucks which I find similar to "empty leg flights" which are sold by private jet operators.