Not anymore, 3.0.4-official was published and has the June security patch level .
You are right. My intention was more like a request to improve the work flow rather than fix the documentation, i.e. only update the information on SPL after actually providing the images with that SPL.
For the record, I had been concerned with respect to FPOS using the 5th of a month to denote the SPL while eOS refers to the 1st of a month. It appears that this difference seems to be harmless with respect to booting on a locked phone:
My FP4 had FPOS with SPL 2025-06-05 (already booted, so the boot loader was aware of this SPL). I then installed 3.0.4-official (reported SPL; 2025-06-01) and was able to lock the bootloader afterwards without bricking. The phone booted without problems.
Warning: As usual, if you want to do this, make sure fastboot flashing get_unlock_ability reports 1 (as in (bootloader) get_unlock_ability: 1) in order to be able to unlock in case of problems.