Ten days is a hell of a long time when those 10 days are an intense warm weather camp plus games. He did train as normal, but didn't take part in the more intense stuff - it's nothing to do with professionalism, Swansea didn't want an injury as much as he wouldn't have done. It's not 'bonding' either. It's training in a more intense way with the team - picking up the sharpness through that and matches.
Yes, he was fit. I don't think Gylfi was blowing out of his arse too much by the middle of September, but he wasn't sharp at all because the training and matches just weren't happening. He'll have been mainly doing conditioning work but as I said, highly doubtful he'll have taken part in much of the rigorous work on the ball that teams do once the conditioning is out of the way.
He did not have a proper pre-season, in any way shape or form. He won't have a 'proper' one this year either, but at least this time him, Gana and Pickford should be coming back into a set up that is well-organised and with a clear identity, as opposed to the s*** show from last year.