That's the kind of conclusion I was coming too yeah, that may be it! The way he says "si" kinda has the U sound with his Portuguese accent so he's taken to just exaggerating it, maybe! SIUUUU!Isnt it the informal equivalent, like Si is Yes, and Su is Yeah?
That's what I've always assumed. I know in French oui is pronounced like 'wee' but you can also say 'way', and that's essentially 'yeah'.