I’ve seen plenty of trucks do the single right hand indicator blink, usually b-doubles and larger.

I interpret that as a few things:

• Old mate has noticed me behind them and has judged the road ahead to be suitably clear, so I can nose out and have a look and go for it if I think it’s ok, and
• That they in turn have a good bit of road ahead of them so they can attempt to nudge all the trailers to the left for me as I come past.