This is partly true.
First of all, if you wish to go this way you need to understand that it's not enough to deny all incoming connections at certain times, you should deny them always. This is so because any time you are connected to the Internet (even if only for few seconds or minutes), you are potentially exposed to malware of all kinds. In fact, if you chose to accept connections only at certain times, you could be exposed to a clever variation of ransomware.
Basically, an attacker downloads a ransomware on your computer. After a while, you deny all incoming connections, but this is pointless. The ransomware is already there, encrypts your data, and asks you a ransom. The point is, to pay the ransom you should use the very same computer, thus opening a connection. In this way, if you pay, you could get infected with other malware, whereas if you don't, you are left with a useless computer (the ransomware could even try to damage your hard drive, or even physically damage your computer, or you don't pay).
There is a reason why "air-gapped" computer are always disconnected, and you should do so.
OK, so you chose to keep your computer always disconnected. This seems great, except it has a few issues.
For example, your computer could become infected with a malware delivered by a flash drive. You might say "Even with the computer connected, I would be exposed to this kind of attack, what's the big deal?". Unfortunately, things are a bit more complicated, for at least two reasons:
if your computer never connects to the Internet, you may be tempted to download stuff on another computer and then copy them on your offline computer via a flash drive (or, even worse, ask a friend to copy them on a USB drive!). Note that you could live with a computer connected to the Internet bit without using flash drives, so you would be exposed to this kind of attack even more when using an offline computer;
things get even worse because you cannot install updates to your OS and/or an anti-virus solution. In any case, you wouldn't be able to update the AV signatures.
A better solution would be to not plug in flash drives, perhaps only using non-rewritable media (of course, checking the files written in the CD/DVD with an anti-virus!). Eventually, you get closer to certain high-security computer setups, which clearly trade off usability for security.
I'm neglecting more esoteric attacks (e.g. jumping air gaps, perhaps through a smartphone), because they are almost impossible to mitigate for an average person, and including social engineering attacks in the "flash drive attacks" category. Physical attacks are always impossibile to avoid, whether you are connected or not.
Bottom line: it is safer, if you don't mess with flash drives and don't let anybody else physically access your computer.