It's kind of like a four-year-old human asking an adult to play tackle football with them. The adult would be confused before redirecting to a more physically appropriate game. Most adult dogs are similarly cautious with young puppies. Once the puppies reach 6 months -- teenagers, more or less -- then the style of play becomes less restrained.
Garry NOT Linda. This is not the way to introduce two unfamiliar dogs. This could've ended very badly. Both dogs should have been leashed and kept at a safe distance and the introduction taken very slowly. Find a Ceasar Millan video to find the correct way to make dog introductions.
A little surprise & kind of shock to watch a rescued dog's behavior/reaction, just like lacking natural instinct. Traumatized mentally? Diffident, as if he forgot what he is ... Time will take care of it, as a hint of it seen.
