Tirthankars are not Siddha while living.

A Tirthankar is a Kevali / Arihant who is still in the body and establishes the Jain Tirth. After nirvana and complete liberation, the same soul becomes Siddha.

So the correct understanding is:

• Tirthankar = Arihant / Kevali in embodied state
• Siddha = liberated soul after nirvana

In Jain Dharma, all Tirthankars ultimately become Siddha after attaining moksha.