I can’t really offer much insight into the reliability of the website you’ve linked, but if you’re really keen to verify what is posted there, you could run your own node, and then find/write some code to index and organize all addresses by their balance. More generally, questions asking about the reliability of a certain website or service are off-topic here.
That said, it’s important to understand what the list actually represents. Namely: this is not a list of how much BTC the top 100 BTC holders own. It is a list of how much BTC is stored in the 100 richest addresses.
This difference is important, as bitcoin users do not map 1:1 onto bitcoin addresses. Many of the addresses on that list are likely the cold storage wallets of bitcoin exchanges, meaning the balance of the address is in fact owned by a great number of users (customers of that exchange). Likewise, some very wealthy users may store their coins spread out across a great number of addresses (in order to maintain better financial privacy, for example).
So while it may be interesting to see what addresses hold the most bitcoin, this metric does not tell us much in terms of who (or how many people) actually own those coins.