You are being downvoted but (aside of your tone) you are at least partially right.

Solving problems is a lot less related to implementing standard algorithms and structures than some people realize. Don't get me wrong, I've regularly had to solve problems that depended on algorithms that I couldn't possibly invent on my own. But let's have a look at the last 5 C and D level problems (skipping the Div 3 and the Educational that I didn't participate in).

1037C - Equalize: An ad-hoc string problem. I don't think there is any standard algorithm at play here. The problem reduces to making a few observations about which operations are ever profitable.

1037D - Valid BFS?: Well, this one clearly already mentions an algorithm: the BFS. It teaches you how it works, but it probably helps a lot if you already have built intuition about that. No other standard algorithm is needed to solve the problem.

1028C - Rectangles has a lot of variations in how to solve it, but the one I implemented did not use any particular standard algorithm. If you count "calculating prefix minimums" as an algorithm then maybe, but that is a stretch.

1028D - Order book: I used a segment tree but that is not at all necessary or even that useful. Most solutions "simulate" the process one way or another, mostly using something like std::set. While it is a data structure, it is not one you actually have to implement.

1025C - Plasticine zebra: Another ad-hoc string problem. Once again, you have to make an observation of what the operation does (or rather: what the operation is in other words). No standard algorithm is necessary.

1025D - Recovering BST: Here you'll probably need to know how dynamic programming works. Intuition about binary search trees helps. Nothing else is necessary.

1023C - Bracket Subsequence: This is a problem with a greedy solution. No standard algorithm is necessary.

1023D - Array Restoration: Once again, I used a std::set<int> for something. In other parts of the problem, no standard things were necessary.

1020C - Elections: Another greedy problem. Or, almost brute force even. Nothing standard to be seen.

1020D - The hat: Well, this one is binary search.

In short, almost none of the so-called standard algorithms or data structures are actually necessary for these 10 problems. In particular, none of the C level problems used anything. Only one D asked you to implement a standard thing, two of them require some knowledge of C++-s library and two of them only require intuition about something.

While it is important to learn algorithms, it is more important to learn problem solving skills. Training your intuition, gaining experience (a lot of the solutions contained "little bits" from earlier problems) and "thinking skills". So, to get to a higher level, just solve problems and don't worry so much about what algorithms you don't know. Instead, learn those on the fly.

you can also look at the tag of the previous C D problems to know what to learn too

Something is fishy here. I did not find a single Div2C with FFT as a tag.

An example:

If you have an array A and you want to find the prefix sums, it can easily be done with advanced algorithms like FFT. Create one polynomial A(x) = A1x^0+A2x^1+...+Anx^(n-1) and another polynomial B(x) = x^0+x^1+...+x^(n-1). The coefficients of A(x)*B(x) are your prefix sums. In particular, A(x)*B(x) = A1x^0+(A1+A2)x^1+(A1+A2+A3)x^2+...

Being a grandmaster doesn't mean you have to boast your skills every time. And he most likely meant div2C and D, not their div1 counterparts.