When people say "Spells do only what they say they do and nothing more" or some similar phrase, they use it for a number of reasons. Though I'll drop the following linklinks as you may find itthem helpful:
- What is the source of the "spells do only what they say they do" rules interpretation principle?
- What RPG concepts does "rules as written" encompass?
Answers thereto these give some excellent origins/explanations/uses for both the phrase, but"spell do only what they say they do" and "Rules As Written" phrases. These two, I believe summarizes it, summarize them best is the following:
[...] The purpose is to facilitate communication about the rules. Historically, rules-as-written approaches to understanding the rules became far more prominent with the rise of the Internet, and that is no coincidence. While RAW interpretations are prone to many, many flaws when it comes to actually playing the game, ideally RAW provides a foundational basis of the rules that everyone can at least agree on. [...]
I think you will also find that people do not put the books onto some sort of Holy Mantle. In fact, I regularly have fun making fun of the rules and their unceasing problems both with friends and just in the TRPG chat room. The reason we use these flawed, broken, human rules is because we really don't have anything else we can use. They are the unifying factor that actually lets us communicate about the game and have a shared basis for constructing arguments and conclusions.
The phrase in question, regardless, does not come across well
This is expanded much more upon in Korvin's answer, so I would suggest reading that. But the way the phrase comes across and is often used is... bad. It can read as "This was so obvious" or "duh" or in any number of similar ways. Ideally, when this phrase is used, people would also explain the rules because clearly, if somebody is asking about them, there is confusion to be found in them. How the rules are saying what they say should be explained in these cases and this often means the phrase could never have been used in the first place.
There's no need to come across as condescending, which this phrase can certainly come across as, and probably does with some regularity. Of course... there's also the rabbit hole of "anything can be condescending" but I'll swiftly avoid that as it doesn't help anybody.