Nah, he's just making a cool entrance.
I kinda agree with this.
I dont think Indio can see from his position whether, Manco has help or not, i think they just wait to see if Manco comes out alive. Mortimer can therefore make a cool entrance.
However even if Indio can see That Manco has help, so what? He might just think its a do gooder helping out. Why would he suspect that they are allies he cannot possibly hear what they are talking about, plus he does not know who Mortimer is.