I believe they were responding to your "confirmation bias, how?" question, rather than the narrative around thermometers. So from that lens, the confirmation bias is choosing to show temperature (objectively measured with assumed-reliable instruments / thermometers), but arguably choosing to hide other kinds/categories of data.
Fwiw, I am not backing them up, since I do not know what other data they might be referring to (and I am not very knowledgeable in topics within climate science).