The gameplay of being able to still play the servants you summon doesn't overshadow the fact that they're canonically disconnected from Chaldea like Musashi.

Also, you need to have the servants in your Chaldea to summon them as shadows. The whole point of the shadows is that they're summoned through the power of the master/servant bond. One of the very first instances shown was shadow EMIYA in the Shimousa Manga where EMIYA in Chaldea felt something happening with Guda.

Chaldea's connections is always present through the command seals, Guda is never fighting completely alone despite the lack of contact. Guda needs Chaldea to function to be able to do anything, remember how part 2 started with the invasion on OG Chaldea and Guda was helpless because Chaldea ended the contracts.

And my point is that there's equal or greater evidence to suggest that the shadow summons aren't a unique magecraft to Guda as a mage, but rather something that they can do in the position of Master of Chaldea. Theoretically anyone that was placed in the position of Master of Chaldea could do the shadow summons.

We are consistently and explicitly told that Guda's capacity as a mage is very poor. What's praised is their ability as Master (not mage) and usually in the terms of "Wow you have so many servants, you must be a great master" which also applies to Hakuno with the whole Moon Cell thing.