What does it mean "to have knowledge"? How much information qualifies you to be a knowledgeable? Plenty of us have a knowledge. Until, for one of your project you have to use Spring Boot and Hadoop at the same time. You know them well, you work with them already. But then you put both libs into one project and something throws an exception somewhere deep down in the unknown code. This is unexpected and does not come from your code. Good luck with searching for solution by yourself. Not that it's impossible, and you would learn something, sure, but troubleshooting takes a lot of time, you'd have to keep your current tasks aside and probability is quite high that someone had that problem before you.

IMHO, what makes you a good software engineer is not an encyclopedic facts in your head but general knowledge about principles, algorithms, patterns, etc. You don't have to remember all methods of all classes of standard library, for details you can check the web or use IDE. But if someone asks you about thread safe objects you have to know what does it mean. What may go wrong if you fail to make it so? Why properties must be immutable? And what the hell immutability is in the first place? All other things you should be able find in the online documentation / language reference.

What makes you a good software engineer is the ability to ask a meaningful questions. We have a powerful tools at our hands these days. Why wouldn't we use them for our benefit? You don't have to remember everything. It is there, within a hand's reach. Sometimes I have a feeling that asking Google the right question to solve your problem should be another skill tested during interviews.

What makes you a good software engineer is the ability to switch the tools and adapt quickly to new environment. Give him Internet, give him StackOverflow and ask him to solve a problem in new language, new framework. If he got the concept well, if he knows principles, that should be more than enough to jump in. Don't get me wrong here. I not going to underestimate guys with years of experience. I am not saying he will write flawless code. But he will solve the problem - that's a good start, isn't it?