I've heard Rabbi Shay Tahan saying that a person doing amida outside the shul, the rabbis say that he like worshipping another god.
What's the source for this?
See Shulchan Aruch OC 90:7. The issue is to avoid giving the impression one prays with his back to the synagogue and treats it and the congregation with contempt but see the details below
Neither should one pray behind a synagogue, if one does not face the synagogue. Behind a synagogue means the side of the opening, which is opposite the side that the community faces in prayer. There are, however, those who understand [the term 'behind'] differently, and it is fitting to heed to both opinions, and even when praying on other sides on the outside of the synagogue one should take care to face the synagogue itself.
All of this applies when it seems obvious that one is turning their back to the synagogue, but if one prays in a house attached to a synagogue and turns one's face toward the Land of Israel, as is proper, and one's back faces the wall of the house which is also the synagogue - this is permitted, because it doesn't appear as if one's face is turned away from the synagogue.
The source for this is in Brachot 6b (translation R Steinsaltz)
Back to the topic of deference for a synagogue, the Gemara records that Rav Huna said: One who prays behind the synagogue is called wicked, as while the entire congregation is facing one direction to pray, he faces the opposite direction creating the impression that he is treating the synagogue and its congregation with contempt. As it is stated: “The wicked walk round about, when vileness is exalted among the sons of men” (Psalms 12:9). In other words, only the wicked walk round about the synagogue in order to pray.