Like many companies, Southeastern Railway are not looking for new staff immediately, so it's a long process.
The most recent round of recruitment (November) did even say " - future opporunities" on the vacancy detail to highlight this.
Also be aware that Southeastern still have applicants from last April in the system and they have confirmed that existing applicants are ahead in the queue.
Southeastern Railway accept national standards for RSSB psychometric testing and do accept recent (I guess 3 years?) passes via other TOCs.
Hope this is helpful.