My idea of a response to this challenge is to have it be part of a general Hapsburg ATL policy in the New World. OTL Spain was awarded the monopoly on the Western Hemisphere as defined in the Treaty of Tordesillas, and settlement was very restrictive, being exclusively Castilians--even people from the other kingdoms Spain was composed of were excluded! Now suppose that Spain is generally allowed to stick to this policy, but with this exception--whenever foreign claim jumpers (as Spain viewed all non-Castilian enterprises) were setting up something near Spanish settlements, the Hapsburg rulers would identify some obscure population undergoing population pressure, and "invite" (perhaps in a heavy-handed, "you can't refuse this offer" sort of way) some of them to form a buffer colony.
I gather that at some point the Bourbons won the dynastic struggle in Spain and managed to break the tie to the Hapsburg system. So for this policy to continue, it would be necessary for the Bourbon intrigue to fail I suppose.
It is not clear to me that the Sorbs were in a part of Germany that would fall under Hapsburg rule, but perhaps this can be finagled somehow. As to why choose Sorb colonists instead of others, someone else has to play that out.
Another consequence of this ATL policy would be that various other border crises would be resolved like this. I presume that with the expansion of English settlement in the Chesapeake, the Spanish would respond with a major non-Spanish colony effort, who knows, maybe Bohemian or North Italian, perhaps Hungarian, settlement between Florida and the Chesapeake. Don't know how forward, that is to say, far north, they'd want to put it--the farther north, they more territory they preempt (assuming the English can't defeat and subdue or extirpate them). So--Georgia? South Carolina? North Carolina?
Might Spanish policy change farther, and become a matter of a bounty put on every unauthorized intrusion, whereby the Spanish Crown gives license and blessing to permanent new colonies by non-Spanish Catholic allies who are invited to invade and crush wildcats and in return get to keep the land they thus blocked enemy powers from taking, out to some limits anyway? So Elizabeth's Roanoke effort collapses too fast, but Jamestown results in some Spanish armada escorting overwhelming numbers of troops from--name any likely Catholic ally of Spain in the 1620s you like--who overwhelm, kill or subdue the English, and are granted a Chesapeake Bay colony of their own, integrated within the Spanish system but autonomously. Then a different expedition crushes the Puritan colony in Massachusetts and that region is home to a second bunch of Spanish allies. Before this the French effort was also crushed with another colony drop (this is a small venture, and involves licensing them to run the fur trade. Basically the Spanish sit inert, but every time some rival takes initiative and tries to accomplish something new, Spain grants a colony to whoever wants it amount their allies in return for the bidding party accomplishing the defeat of the enemy colony.
Of course this is self-limiting--if England is trounced at Jamestown, what are the odds that the Plymouth company tries for a colony in the north? If New France is nipped in the bud, against whom would the Spanish want to settle a blockading colony in Texas? To put the Sorbs there, someone anti-Spanish has to try and set up their own Texan colony first, and it is unlikely since such a colonial effort would have to sail into the Gulf of Mexico to arrive at Texan shores somehow. OTL the French could do it, at least with support from earlier New France upriver. Indeed if Spain is losing her grip and some of these colony drop counterattacks might fail, then they might revert to the idea of a buffer colony, so that Texas is Spain's answer to Louisiana.
How the Sorbs come to be one of the colonies is still a mystery to me, but someone else might suggest the connection.