The "new" shortcut is a really cool finding! Good job going through the resources again for this game and picking up on this secret that had been overlooked until now (at least by speedrunners).
The shortcut itself is one of those "impossible" secrets that I wonder if anyone managed to find legimately on their own by just playing the vanilla game and equipped with the manual, but without exploring save states or looking at the RAM or in the code. I guess we'll never know, but I'd be very interested to hear what the developers for this game and Solomon's Key had in mind with some of the overly obscure secrets. Maybe the sole purpose was to sell guide books?
About the TAS itself. The new TAS looked the same as the previous one up until palace room (PR) 4 and then the same again from PR 12 until the end. PR 4 and 11 play out slightly differently this time around because of the shortcut, but the changes made sense to me.
The shortcut traverses the level 5 corridors and the end of the last level 10 corridor (
https://www.vgmaps.com/Atlas/NES/MightyBombJack-Pyramid.png ). There are some routing decisions when going through these corridors and which platforms to jump to (shorter jump now and then a bigger jump or bigger jump first and then a shorter jump?). I checked one of those cases, but couldn't find an improvement by taking the alternative route. The game mechanics are not trivial when testing, so I didn't check all of the alternative routes. In the end, nothing stands out to me as questionable, just noting that there are sometimes route alternatives that are not obviously slower just from watching.
A question that came to mind when hearing about this shortcut was if the conditions were really exactly as described or if a different setup could be found.
This sub-routine runs in corridor 5-5 (with reference to the room numbers in the link above):
A8E9 LDA $0396 = #$00
A8EC BNE $A925
A8EE LDA $0395 = #$00
A8F1 CMP #$02
A8F3 BNE $A907
A907 LDA $03CE = #$E9
A90A BPL $A925
A90C LDA $0392 = #$5D
A90F CMP #$5D
A911 BNE $A925
A913 INC $0392 = #$5D
A916 INC $0395 = #$00
$396 - unknown purpose
$395 - death counter in corridor 5-5
$3CE - MBJ status flag
$392 - room-specific death event "trigger"?
The first four bits in $3CE indicate whether MBJ is alive (0) or dead (0xE). The last four bits don't matter for this sub-routine, but indicate whether he's on the ground or in the air (only a quick check, so maybe there are other possibilities too?). So in the example above, MBJ just died.
I didn't look into $392 in detail, but it's set when dying in corridor 5-5, which allows the check in the sub-routine to be passed. If I remember correctly, there are other secrets that require deaths in this game. Maybe this address is used for them as well?
Each time you die in 5-5, $395 is increased by one and when it's counted up to 2, the sub-routine branches and $3BC changes value, which opens the (ultra-)secret passage to 5-X.
So let's get back to how this fits with the setup described for this shortcut:
1. Collect the sphinx in 5-2 - This opens up the passage to 5-5. There is no other known way to open up this passage.
2. Enter 5-5
3. Die in the fire pit - There are no enemies spawning in this room, so the only way to die is in the fire pit (or maybe by letting the time run out?). Dying in 5-5 increases the value in $395 and the sub-routine that increases $395 only runs in 5-5 (at least of the rooms visited in this TAS). The only other times the value in $395 is updated during the TAS is after clearing a PR and then it's to reset its value to 0. So maybe this address is used for other secrets as well and resetting it after each PR ensures that you don't start triggering a secret in one level and then trigger a secret in another level earlier than intended?
4. Repeat 1-3
5. Repeat 1 and 2
6. Enter the passage to 5-X
Without a deeper analysis, it's of course always possible that one can access the relevant code sections through different parts of the code, opening up new possibilities. But as far as I can tell and with today's knowledge, the code seems to support that the steps in the described setup are indeed required to open up the shortcut.