Ticketsatwork.com is a legit site. Could be your company is in the system from a previous partnership or not everyone in HR is aware of it. Did the person you spoke with in HR deal with employee benefit programs? Not all HR does the same services so it's possible they may not have known. Our HR had outsourced benefits so most in the team only knew about discount programs as an employee would, if an email came out about it.
I'd check the ticket price all the way to the end of checkout, I know they have shipping and other fees added in that's not directly advertised. Lately after the fees and whatnot I find they are just about the same or a little higher than sites like
UT. I haven't actually bought tickets from them in many years, but I know people who use them for hotels. Sometimes they've been the cheapest non-refundable hotel rate around even with the fees.