I would even ask "what is your definition of internal martial arts?". I say this because even Southern Bagua is referred to as internal afaik. I think people get wrapped up in appearance when it comes to that concept sometimes.
First that term is a post WWII (I think 1960s?) Artifact. Second if I am tense and someone catches it, they call me out. I need to be relaxed to properly do Wing Chun. Maybe the problem is some people don't realize there is actually a debate, in side some portions of the Wing Chun Community, as to whether we practice an internal, external, or "all of the above" martial art? Simply because you move in "straight" lines doesn't mean you universally follow stereotypical external principles.
I did Xingyi and Bagua for about five years, with some Taiji mixed in.
Some practitioners of these arts refer to them as Neijia arts, or internal. My instructor in those arts said that he regarded them as "internal" because they were based on the principles of TCM - Xingyi on Five Element Theory, BaguaZhang on the eight trigrams (involved in some acupuncture systems) and taiji on yin and yang. Where as other arts such as WC are not ... in his opinion.
"Internal" is not the same as "soft". Xingyi can be as hard as a fist of diamond.
The whole "internal/external" thing is overblown and at its base, marketing. Many WC practitioners claim they do "internal" training, but few of them explain how other than vague references to breathing and softness. Stonecrusher69's recent video on the subject is an example. No disrespect, but there's little hard definition in there.
Jiu Jitsu done properly is more "internal" by my definition (and it seems we can all choose our own, doesn't it?) than TWC. Tim Cartmell, who is highly ranked and experienced in both Neijia arts and BJJ, has stated similar opinions, and that in his opinion BJJ is an internal art.
I got introduced to Stanley Tam, a BJJ black belt from Beijing, at a Steve Maxwell seminar. He also has developed and teaches a system of Qigong and Chines Natural Healing, and he is teaching these to Steve. Very interesting guy ... I venture he knows more about the "internal" stuff than many Kung Fu guys.