Glazes are all different, and it's not entirely out of the question that glazes across different manufacturers or even in different lines from the same manufacturer are somewhat different.
Glazes do contain small amounts of metal oxides of sodium, potassium and calcium, aluminum, iron, lead, copper, cobalt, tin, zirconium, even lead (but not for food these days.) Nearly any metal you can think of has been used in a glaze (including uranium to make it glow(!?). All of these have the potential to react with a strong acid or base, so it's not entirely surprising that some glazes etch. I suspect that glass, which also contains trace amounts of metals for various reasons is also etched for the same reason.
Contemporary Pyrex is actually tempered soda-lime glass which contains alumina and other metals. As a matter of fact it contains a surprisingly large amount: 13% Na2O, 10.5% CaO and 1.3% Al2O3. (Ref: Seward, T. (2005). High temperature glass melt property database for process modeling. Westerville, Ohio: American Ceramic Society.) Older Pyrex manufactured in the US (and reportedly currently manufactured outside the US) was originally borosilicate glass. The official trademark of the borosilicate product was PYREX, and when the Corning sold/spun off the line the trademark pyrex or Pyrex was used. It's reported widely (but unsubstantiated) that if you find older PYREX marked glassware it is borosilicate.
Boron is a metalloid, not a metal (as is Silicon). Metalloids generally are less reactive than metals, so the extension of that is a glass lower in trace metals will be less damaged by strong caustics.
Anyway - the admonition not to use "Pyrex" is because several have shattered (and I think people here have reported that?). Soda glass has a much higher coefficient of thermal expansion than borosilicate glass (it expands more when heated). If you combine that property with micro-etching, you have lots of points at which a fracture can start. remember, "cutting" glass is really making a small scratch (etch) and then using force to break the glass along that line. Throw in the cooler top of a measuring cup and the intense heat of hydration lower when you add lye, and it's no wonder Pyrex measuring cups have shattered.
All that is to say etching per se is not necessarily bad. Ceramics do not rely upon the structure of the glaze to provide integrity (where glass is technically 100% "glaze"). If it is etching, I'd give it a definite shrug of the shoulders and go on about my business.
