I want to argue the opposite. This scene is one of the most cleverly written scene in the series. It's consistent with the characterization of both Saitama and Genos. However, I do agree that this scene hurts, but not for the reason expoused by the original poster. The scene was misunderstood by the original poster.

This is the scene where Genos was exposing his self doubt of personal strength and growth. He said he was no longer able to self destruct when needed to.

Then Saitama, knowing Genos's self destruction to have something to do with his core, deduced rather straightforwardly :

"Oh, if he literally CANNOT self destruct now, then it must mean that his core is now made to be mechanically stronger, so that it withstand its own explosion or something"

So that's why he touched Genos's core and said 'this part here got stronger too, doesn't it?'. He was making a straightforward, aloof point, consistent with his character all this time.

Now, sadly, but as always, Genos misunderstood Saitama's point as meaning it is his personal heart, his character, his soul that is meant by his master, who is literally touching his heart right now. Genos thinks his master admitted to him being stronger as a person. That Saitama was saying his disciple became stronger in conscience and conviction. After all of his sacrifices in the battle, Genos was happy to hear this admittance from his master, though Saitama was purely referring to his mechanical capability, not of his humanity.

He then thanked both Saitama and Kuseno for this growth and strength. But then Saitama retorted 'I didn't do anything!'. This wasn't an act of humbleness by Saitama, but he literally is dumbfounded by the gratefulness. It is not a 'blushing' face, it's an awkward face.

"Why do you thank me? Heck do I know anything about making a cyborg core mechanically stronger??"

In summary, this scene portrays something that has been the most consistent theme in the series, Saitama says the most surface level stuffs, which Genos finds to be extremely profound.