What do you mean by “reliable?”
Despite what you might see on Quora, intelligence is a fairly well established construct in psychology. Entire courses are taught in grad school (e.g., here’s a sample text: https://www.amazon.com/Contemporary-Intellectual-Assessment-Fourth-Theories/dp/146253578X/ref=mp_s_a_1_7?crid=2GQJIRO53ISL5&keywords=intelligence+dawn+flanagan&qid=1657628214&sprefix=intelligence+dawn+flanagan%2Caps%2C66&sr=8-7). And scores from established tests are used diagnostically (eg, to diagnose intellectual developmental disorder and rule out many competing diagnoses in comprehensive assessments.)
Understanding the statistical definition of reliability, knowing what validity means, how scales are constructed, and how to evaluate their use is typically well beyond what we can convey to the public. Yet these are foundational to evaluating the merit of intelligence (or really most constructs in psychology). This creates kind of a disconnect between experts and the layperson.
Your question, are there intelligent people who score low on IQ tests, rests on two different definitions of intelligence. The intelligence measured by a test is psychometric intelligence. This correlates with, but is not the same as, other types of “intelligence” that we talk about in everyday language, like emotional intelligence or creative intelligence. You can be quite emotionally savvy and have an average or below average IQ. What we tend to see is that these traits go together, though there are of course exceptions (there are some studies that show that people with extremely high IQ may struggle to maintain social relationships.)