Repeatable Read Isolation Level "transaction start time"

https://www.postgresql.org/docs/16/transaction-iso.html#XACT-REPEATABLE-READ

The PostgreSQL documentation for the Repeatable Read Isolation Level states the following:

“UPDATE, DELETE, MERGE, SELECT FOR UPDATE, and SELECT FOR SHARE commands behave the same as SELECT in terms of
searchingfor target rows: they will only find target rows that were committed as of the transaction start time.” 

What is defined as the "transaction start time?" When I first read the statement, I interpreted it as the start of the
transaction:

BEGIN;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

But in my testing, I find that according to that statement, the transaction start time is actually "the start of the
firstnon-transaction-control statement in the transaction" (as mentioned earlier in the section). Is my conclusion
correct,or am I misunderstanding the documentation?