I'm a microbiologist. I can speak from experience (my grad research required attempting this a few times) that entirely sterilizing anything of microbes is incredibly difficult regardless of technology level. They are tenacious little fuckers. I'll lay this out for anyone interested.

Gotta Kill 'Em All: Most microbes are fairly easy to kill using simple physical and/or chemical means. Some are more difficult, like spore formers, bacteria that produce little personal suspension pods when conditions are rough.

What matters is you start with huge quantities of microbes, they're everywhere, and you can't see them. All you need is one to survive to potentially reproduce into vast legions of descendants. Even NASA's protocol is about lowering the total number, thereby reducing, not eliminating, the probability of causing an issue. Miss the wrong microbe in the wrong environment and you've inoculated a planet.

Checking Your Work: How do you verify that you successfully sterilized your tool? You might say culturing - swab it and grow that on some type(s) of media. That's NASA's protocol! It's just not very effective.

Not all microbes grow on all media. There are an estimated one trillion microbial species on the planet and we only know how to culture less than about 0.5% of them. The rest are a mystery, largely uncharacterized*. Most sterility testing is for known microbes of consequence, not every microbe in existence.

Microbiology is very often a science of slapping your tool or workspace and exclaiming "good enough!", not absolute precision and 100% efficacy, both of which are practically required if you want to be sure you don't inadvertently pull a "smallpox blankets from space".

*Fun fact: Sometimes people get sick with something atypical, that doesn't get IDed through standard testing. I worked for a time identifying these pathogens via gene sequencing. There was a whole lot of "that's a new one" out there.