If you take a student (someone asks to learn from you), you’re a sifu. Whether you’re a good sifu or not is an entirely different matter, but either way you’re a sifu.
It’s like having a child. You may be an experienced individual with decades of child-care behind you before you have a child, or you could be barely more than a kid yourself with no idea how to care for anyone, but either way, you’re a parent.
Fighting doesn’t really matter. Some people never studied WCK, some never studied any martial art, and can fight incredibly well. Teaching quality also doesn’t matter. It’s a completely separate skill set that requires specific and dedicated training all its own.
So you could find a clueless sifu who fights up a storm but can’t teach worth a d@mn. Or one who knows some, can fight some, but can teach you really well. Or one who knows a ton, can’t fight out of a paper bag, and struggles to pass things on. One in a million might actually know it, use it, and be able to pass it on to wide range of students with differing needs and learning styles. Grab those when you can, or grab the best total package you can get to consistently enough for it to matter (if Yim Wingchun herself offered to teach you but only for 15 min. every second decade, your progress might be slow).
Sifu is just a word, a martial/religious embodiment of the mentor/teacher/parent relationship, or an honorific applied to anyone with any kind of skill (driving, cooking, chess, etc.)
Best to look beyond the skill to the person behind it. Are they someone who can teach what you need, are willing to teach it, are able to teach it in a way you can pick up, and is consistent enough for you to make progress (bonus points if they’re decent human being as well).