I wonder what they will think of sigmar
That's actually a pretty serious weakness of the Admech I'd say, they'd be far far less able to navigate questions of faith with subtlety. Certainly they can knock over various places with cool weapons and establish industry in various ways, possibly even get people to help them in various ways too, but they have the lowest possible 'social stats' as it were. They're the most alien and the least flexible, even the Magos Explorators.
Comparably, the Administratum and Ecclesiarchy have existing functions dedicated to starting or bringing colonies into the fold again, for example for the later:
"The Battle Sisters of the
Orders Sabine accompany the
Missionaria Galaxia on missions to rediscover human-settled worlds and to prepare them for re-integration into Imperial society and culture. These Battle Sisters specialise in infiltrating primitive societies and introducing elements of the
Imperial Creed to the natives."
Now the Missionaria Galaxia will have operatives in an Admech fleet in case of situations like this, but the Admech still aren't particularly personable.
Imagine, for example, you're some regional noble fighting Greenskins and suddenly the Imperials arrive. In one scenario what appears to be a tide of mechanical creatures run over the greekskins. They ride on a bizarre assortment of mechanical creatures and wield horrific weapons. One of these robots shudders up to you and announces that the Omnisiah has sent them to help. What's the Omnisiah? The God of the Machine. Then they leave. It's all very strange and they're so weird you might even suspect some sort of sorcery is afoot.
Comparably, say your rescuers are great knights, armoured and tall, wielding magnificent (but recognisable weapons). One of them takes off his helmet and addresses you, he's incredibly handsome, superhuman surely! He speaks your language well (you don't know he's been eating brains to learn it) and you have a conversation in which he mentions that he too worships the same gods you do, incarnated in an impressive figure who he's apparently related to. Indeed he claims descent from one of Sigmar's siblings, which is a fantastical story but still very impressive. He seems to have some sort of innate charisma to him and you're happy to assist him.
Now even though Space Marines aren't usually deployed for diplomatic reasons, they can do it. They're superhuman all the time, not just in battle. There's not that many of them but they could quite swiftly unify the Border Princes for example if necessary simply because they're so impressive. Even if they don't have great knowledge in terms of scholarship or whatever, again they're superhuman, they can adapt and strategise.
To answer your question though, the Admech would probably perceive Sigmar as a curiosity, esp his apparent ability to give 'magic' out and the whole phenomena around that, but I don't think they'd care much. Apparently they managed to synchrenise with the Squats' ancestor worship, but there would be some pretty inevitable tensions given the lack of social science magi in the admech.
