In Texas, liquor (spirits) sales and non-spirits sales are different classes of licenses with different rules. Costco in Texas does NOT sell any spirits/liquor inside of the membership area, only beer, wine, and premixed cocktails (e.g. margarita in a bottle, which is made with agave wine not tequila anyway). The attached non-member liquor store is the only part which sells spirits, and as required by Texas law is open to the public without a membership.
Related to this, Texas also has "blue laws" which ended in 1985 other than the parts covering automotive dealers and liquor stores. So in Texas, liquor stores cannot be open on Sundays (nor can car lots). This is another reason Costco may be incentivized to not sell liquor within the member-only part of the store separately from the other requirements from TABC, because they are open on Sundays.