The source of the "hard yellowish coating" may be Silver carbonate (Ag2CO3), as per Wikipedia:
Silver carbonate is yellow but typical samples are grayish due to the presence of elemental silver.
I suspect the teapot in question is "electroplated Britannia metal" per Wikipedia with a nickel alloy as the base metal, as to quote:
After the development of electroplating with silver in 1846, Britannia metal was widely used as the base metal for silver-plated household goods and cutlery. The abbreviation EPBM on such items denotes "electroplated Britannia metal". Britannia metal was generally used as a cheaper alternative to electroplated nickel silver (EPNS) which is more durable.
The Baking Soda will interact with any soluble Ag ions to form the unstable Silver bicarbonate which breaks down to Ag2CO3 on warming.
The question is what is the source of Ag+?
My speculation is that the Al foil was completely coated given the surface area of the teapot, and the nickel-based alloy is notoriously cathodic (hence, the use of Nickel alloys in coins). So now, the Silver itself can be subject to an anodic attack in the presence of H+ (from amphoteric NaHCO3) and O2 (from air exposure), resulting in Silver ions.
This was a somewhat unique situation, hence the successful use of the Al/NaHCO3 process on the Silver-plated flatware.