Do you believe what you read or what you see? I’ll leave you with that question.
I believe in evidence. Evidence that is positively and solely in support of one single conclusion over all over proposed conclusions. Evidence that is established by objective measurement in a verifiable, ie. repeatable manner, using the best tools available to make such claims.
That evidence is that, as is multiply attested to by several and independent professionals using equipment calibrated for the sole purpose of determining the various attributes of display quality, the Galaxy S9+ is nearly twice as bright in direct sunlight, as the iPhone SE, when both are using the settings that allow for the maximum brightness to be achieved.
In order for your proposal to be considered for even the possibility of being correct, you would have to provide one of two things. 1) Evidence that each and all of the multiple sources that verify the standard claim are false and that either their conclusions are intentionally misleading or that they all make the same mistake in measurement. 2) Measurements of your own, of a superior nature than that of the experts using professional equipment, that contradict the preponderance of evidence, which all aligns against your case.
So our choices, given the available information, concerning your position are as follows.
1) You are lying or trolling. We're assuming that this is NOT the case, because to assume it is the case would be rude.
2) You don't understand the evidence and therefore it would be reasonable for us to try to help you understand the reality and if that reality doesn't agree with your perception, help you troubleshoot the issues preventing concordance.
3) You are mistaken in your ability to repeat the conditions that enable the maximum brightness on the S9+
4) You have a defective device - you claimed to have ruled this out by testing multiple devices.
5) You have defective eyesight - so far there is no evidence to support this claim, so occam's razor... let's not introduce this one.
So if we cancel 1, 4 and 5 based on your narrative - then 2 or 3 are the only logical conclusions, unless you have a 6th proposal - but be cautious, because if your 6th proposal is that all of the experts are wrong because they don't agree with your anecdotal experience, that is a positive claim which requires evidence to support not only it's reality, but even it's possibility, which would require precedent. The rule in effect there is that all positive claims must be supported by positive evidence. Otherwise they are merely assertions, which are unworthy of consideration.