So the short of it is the rules are poorly worded, in many different locations, and GSC behaves inconsistently with how most other armies behave leading to interpretation problems. Also please note I am going to play devils advocate on this as you specifically asked to try to see it from the other side. Note I agree with you. Though I will say I have seen groups rule it like yours seems to, and to be honest I can at least SEE their reasoning even if I don't agree w/ it.
So you have the right of it that technically GSC units picked up from ambush can arrive via "underground" turn 1. It is basically the same as ork's psyker Da Jump spell, and you should point to that as to why this shouldn't really be argued against you. Whether or not this stays though I think remains to be seen, it wouldn't surprise me if GW walked this back to say you can't use the pull off the table strategem to arrive turn 1 underground. With the psyker stuff or when its done via relic its either once per game or once per round, so people don't generally care to much. GSC kind of get to cheat this by getting 3 DS turn 1 w/o relic or having to pass a psyker test. In addition the unit that will arrive turn 1 is never vulnerable as its off the table by the time your opponent gets to shoot even if they go first. GSC is in an fantastic spot currently getting full access to guard as well as lots of supplements to handle things guard doesn't do well, turn 1 DS is icing on the cake.
As far as why you are getting push back, I think a lot of people look at it and say that seems unreasonable since you aren't placing the model on the table at any point, even though the rule states the maker counts as them on the table. GSC as a whole is a really different army, and functions extremely oddly between deployment and first turn shenanigans, making it hard for people to reason through. In addition there is the vagueness of the statement for the purposes of tactical reserves, one could read it as for the purpose of being able to DS units. Without that wording GSC wouldn't be able to only deploy via blips as they would have nothing on the table. The wording on the errata unfortunately isn't specific enough. The "for the purposes" part of what I think leads to people making a poor interpretation of it. If it just said GSC units in ambush are on the table then there is no room for interpretation. You cut out the part in your re quoting on the FAQ that is probably causing the hang up.
When setting up your army during Deployment for a matched play game, at least half the total number of units in your army must be set up on the battlefield, and the combined points value of all the units you set up on the battlefield during Deployment (including those that are embarked within Tr ansports that are set up on the battlefield) must be at least half of your army’s total points value, even if every unit in your army has an ability that would allow them to be set up elsewhere.
Likely people are interpreting the "for the purposes of" applying specifically to the requirement of having half your army (points, power, units) on the table, even though the unit isn't on the table. People are likely looking at that and saying then they aren't actually on the table which wouldn't exempt the from the no DS turn 1.
Combine that with the "no longer in ambush" part of came from below which suddenly removes their "for the purposes of" protection with a little rules lawyering, and suddenly they were never on the table, don't have protection from the "for the purposes of" and models not on the table can't arrive turn 1. Its not that that hard to follow their thought pattern IMHO, its wrong.... but it isn't completely illogical. At the very least you can see how they view the correct interpretation (IMO) as a rules loophole.
Typical poor wording on things from GW causes your problem.
Between the you have to stay 9" away from me on your turn bubble, the I can remove up to 3 markers, i can pick up 3 units that were supposed to be revealed and now they can DS on my first turn, and getting to include guard the army is very bizarre. I happen to play against a GSC player often so I am quite familiar but everytime some 1 new has to play him its an ordeal. The general consensus he gets from people on it even when he finally convinces them is that its abusing a loophole in the rules. While that isn't the case its not hard to see that point of view if you make a little effort.
IMHO I wouldn't be hugely surprised if GW messes more with this rule in a future FAQ GSC have a lot going for them, w/o 3 turn 1 DS for 1 CP but time will tell.
Edited by GrinNfool, 07 April 2019 - 05:58 AM.