My personal experience with ISFPs is that they do hold back a lot. I haven't met too many who I'd call strong verbal communicators, especially when it comes to their feelings. Most I know in person will reach a point where it comes out in a passive-aggressive manner, via snarky comments or destructive behavior.
Like any Fi-dom, they may be more open with those they trust in their inner-circle. The theoretical descriptions of Fi describe this reservedness as typical also. Jung describes a person who does not venture to express much of themselves unless it's a BIG DEAL to them, and a big part of this is due to an acute awareness that you feel differently from others & will not be understood. There's also little desire to affect other people with their feelings, whereas with Fe types, they very much want to influence others in order to create consensus and unity.
Some of the most quiet, reserved, withdrawn people I've known have been ISFPs. I think extroversion/introversion levels in terms of demeanor is an individual matter.
I think enneagram may play a huge part here too. Many ISFPs are enneagram 9 and they will withdraw & shutdown to cope, avoiding conflict & communicating with others. Profiles tend to be based partly on theory, partly on the writer's own observations. I often see many INFP profiles, for example, read like 9 types, and as a 4 I don't relate as well.
When it comes to INFP profiles, personalitypage is one of the better ones, IMO. That's the one which made me first consider I may be INFP, since I usually test INTP.