Welcome, Sherlock!
I don't believe Wendy was giving you any attitude or intending to insinuate that you didn't know how to install a carseat. Unfortunately, communicating online tends to lose things like facial expression and vocal cues that can differentiate between something coming across as friendly or accusatory!
Without putting words in her mouth, I would venture to guess that Wendy replied as she did because statistically, most carseats are not installed properly (up to 90% have an installation error of some sort,) and our top tethering rate is abysmally low. Google tells me that our top tether rate in the US is between 28 and 47%, which means that you are actually in the *minority* by using your son's tether! Terrifying, isn't it?
A child in the center has a 40% safety advantage over a child seated outboard, but a rear-facing child has a 500% safety advantage over a forward-facing one. So, we consider rear-facing children the most protected - then forward, boostered, and seatbelt-only. Older kids are more protected than younger kids, since their skeletons are more ossified, hence the two options Wendy gave. If your son still fits in his seat rearfacing, it would be a safety advantage to turn him around, but at over 2, he does meet the currently recommended minimums for forward-facing.
Looking at all of the above, that's where Wendy came up with her recommendation to put your forward-facing toddler in the center. Are you using LATCH for that install, or a seatbelt? Then your rear-facing infant can go outboard, balancing the safety advantages for each child. If the seats don't fit side-by-side with independent installs (such that if you remove either, the other stays tight,) then you can put them both outboard - either side is fine for either kid.