I'm surprised you're almost sticking up for Ledger here, as you usually are quite critical of them if I'm not mistaken--but you're right.
It's a reasonable position (and indeed, the correct position) to be critical of a company when they have obviously failed (such as in Ledger's database breach), but not jump to conclusions without evidence regarding other issues (such as in this case). Literally every time I have seen someone complain that their hardware wallet was hacked, it has turned out to be user error, not following the set up guide, insecurely backing up their seed phrase, entering their seed phrase on a website, etc., and no fault of the hardware wallet or its manufacturer.
He bought the device in December and in February spent some BTC on the wallet. Considering it was not his first transaction, why the wallet wasn't hijacked before February? Personally, I think it's a human error.
How do we know it wasn't his first transaction? Regardless, lots of people with hardware wallets make a small "test" transaction first, and many more use it to slowly build up their funds over time. If the attacker knew the seed phrase from the outset (as they would in the case of a pre-initialized device), then it may well be in their best interests not to clear out the wallet immediately, but to wait a few months for a larger amount of coins to be deposited. Or perhaps the attacker only just discovered the seed phrase because OP had entered it on a website, uploaded a screenshot to a cloud backup, or something similar.
On a side note, I understand that a hardware wallet if fully air-gapped is secured, but I'm now wondering, does plugging your hardware wallet to your online PC or laptop necessarily pose a risk of whatever degree to your funds? That is, even if you are only operating on your Ledger Live, for example, for the entire duration?
By plugging your hardware wallet in to a computer with an internet connection which is filled with malware, the worst thing that can happen is that some malware creates a transaction and pushes it to your hardware wallet to be signed. Without you physically approving the transaction on the hardware wallet itself, then the transaction can never be signed and therefore never be broadcast, and your coins can not be stolen. The only way coins can be stolen in this manner is if you are completely careless and approve a transaction on your hardware device that you didn't create and without checking it.
How was this information obtained? I mean that the boxes are not sealed anymore.
From Ledger themselves:
Ledger deliberately chooses not to use anti tamper seals on its packaging. These seals are easy to counterfeit and can therefore be misleading. Rather, genuine Ledger devices contain a secure chip that prevents physical tampering: this provides stronger security than any sticker possibly could.