My understanding is the originator of the comment is rather concerned on pipes material in your system, refers to possible combinations API 5L Gr B- A234 WPB vs higher grades API Gr X60- A860 WHPY 60; in the end of the sentence gives a reason (which would be better said vs the concern) as "fitting to be selected as high strength fittings in sour service are prone to SSC".

In practice SSC is frequently associated with welds- this may be the root of concern on mixing materials, you risk to develop SSC in the connected pipes, even the tee material selection is correct.

Sulfide Stress Cracking is a form of cracking mechanism in contact with sour/ wet H2S. The qualification of material is by NACE MR0175 for Oil&Gas applications. I don't know your case needs NACE qualification.

In fact today NACE describes the mechanism of SSC as either type I SSC (SOHIC) or type II, typical hydrogen embrittlement and the details are quite complicated. Into these complex details/mechanism theories one can find an explanation of the practical fact that SSC is prevented (but there are limits of prevention, you can reduce the risk as acceptable vs H2S amount) by minimizing the hardness and strength of the alloys used in wet H2S systems; this is accomplished through material selection, weld procedures and PWHT.