It could be. You can always call the plan administrator with any questions.
What Is a Thrift Savings Plan (TSP)?
A thrift savings plan (TSP) is a type of retirement investment program open to federal employees and members of the uniformed services, including the Ready Reserve.
While the TSP isn't technically a 401k, it is a defined contribution plan just like a 401k (and a 403b for that matter). Defined contribution means your employer – in this case, the Federal Government – will contribute a defined amount toward retirement for you subject to certain rules.