There was a partial blockade, not a full blockade, and this partial blockade came after Palestinians launched the second intifada. Prior to the october 7 massacre, perpetrated by Hamas and gazan civilians, tens of thousands of gazans were able to travel out of gaza through egypt and israel, where many of them worked. nearly 75,000 truckloads of food and cargo went into gaza from israel in 2022. Gaza exported lots too.
I don't quite see how under those circumstances, they were able to build "a more powerful army than many European countries", unless you talk about Luxembourg or the Vatican.
In terms of Hamas's army being more powerful than that of many European countries, I'll respond to that below.
And the Wikipedia article you cite has been manipulated by a band of ideological editors and is not reliable, and has no value (inverse value?) as a citation.