He used the Teserac(space stone) to get to Vormir, but couldn't get the soul stone from there and instead was relegated to being
Charon of the Soul Stone. Iron Man did lose Spiderman, so maybe just suffering a great loss was enough to qualify him. In his mind, it was by his own hand as he gave him the suit and he died just like he said he would. This would also make sense in regards to the one path Strange saw, forcing Tony to be saved.