That’s not not how the concept is applied in halacha. If there is a machlokes between contemporary Poskim regarding whether or not something is assur d’oraisa or mutar, one is allowed to be someich on those Poskim who allow it. Those poskim hold that there is no safeik – they hold that it is 100% allowed- so I can be someich on them.
An example of a Safeik D’oraisa l’chumra is as follows: Reuven is unsure if he bentched or not. Bentching is D’Oraisa for a man, so he has to bentch again. The safeik does not revolve around a machlokes haPoskim; it revolves around the “metzius”.
You are correct in that there are people who are exceeding careful in halachic matters and are never someich on the more lenient Poskim whenever there are more machmir opinions in case the more machmir Poskim are really right, but that is considered “being machmir” and is not required halachically, and may not even be the proper approach for all people in all situations. And certainly, one can not judge someone else for not choosing to be machmir.