Kira and Aaron were rather baffled when they heard him say that with ease.
Until now Shin had been saying that his healing skills wouldn't work so they expected him to show at least some discomfort.
Instead of that, he seemed quite confident and they had no idea about the source of his confidence.
But Kira knew that Shin wouldn't just lie. Aaron was also not suspicious of his saying.
For some reason, after talking for some time, they thought that they could trust Shin.
Also, since the moment they had started calling him 'Master Shin', they had automatically developed a respect for him and that didn't fade even by a bit.
Of course, Aaron wasn't able to control himself earlier and was off to hurt Shin, but he was now calm and was able to comprehend everything properly and calmly.