Is this a legit steam admin?

You're never going to chat with anybody who has administrative access in Steam, so for all intents and purposes the statement holds true. Valve as a company almost never interacts with the community, and when they do, it's not in chat. The personnel working for Valve also, by and large, do not oversee or police anything going on within Steam, like you would expect from an "admin". There are a lot of administrative things they could do - and I think they should be doing, such as policing what gets published into their store for abuse - but they don't even do that. Anybody approaching you and claiming to be a Steam admin is definitely a scammer.

A large portion of scams right now center on inviting someone who claims to be "Steam admin", then either convincing the victim to give up their username/password or trade away their items. Scammers doing this are always coming up with new ways to convince victims of their legitimacy (e.g. fake games to create their own profile badges), and it'd be impossible to list them all. They also thrive on any kind of confusion or mixed message, going so far as creating their own "official" Steam guides filled with misinformation about Valve or Steam are, who oversees it, how trading works, and what is allowed or expected of traders. Getting into semantics and adding a bunch of caveats like you suggested will only confuse the younger and English-second-language traders, who are already the primary demographic falling for it. There is no way a 7 year old player from Romania who doesn't even know what "Valve" is will read or understand what you wrote. Seeing that Valve will never speak to or interact with you directly, stating "there's no such thing as a Steam admin" is the simplest and most concise way of telling people "it's a scam", without sending a mixed message that scammers will most definitely capitalize on.

Their is no such thing as a steam admin.