They might not know that there are resources online that they can use, as they may have been only given company arranged training, and never encouraged to look beyond for anything to learn. As such, I wouldnāt let it automatically be a red flag, but lead it to more probing to understand what has made them not do any independent learning.
Were they never given time at work, and outside of work they have other commitments to focus on? As you wouldnāt want to discriminate against those who try to keep a work-life balance with children or other activities.
Did they previously try and found they were shut-out from trying new things, and after a while they decided to stop bothering? As it might be what has lead them to looking elsewhere and being interviewed by you, and they need to re-ignite that passion again.
This is very true. Iāve been in testing for more than twenty years in a number of roles; I never knew there was such a thing as the testerās community until I started my current role fourteen months ago. (And Iāve learnt more from the community and fellow testers in those fourteen months than I probably did in the preceding twenty years!)
Yeah, itās a maybe and a bit. Not automatically a red flag. Interviews are opportunities for interviewers and interviewees to discuss and elaborate on context and as you said can definitely lead to probing to understand. 
I found this in an article (exactly where escapes me now), but itās brilliant and has assisted me in setting out interviews for testers many times.
Brilliant thanks! Just couldnāt remember where I got that from, now I can revisit And thanks @danashby, itās been a brilliant resource!!