Before answering the question, let me clear up what appears to be your misunderstanding about what we wrote in a recent paper about Gaza. We wrote that if Gaza is properly viewed as an independent state, the border between Israel and Gaza is an international border. We addressed separately the question of whether one can credibly argue that Gaza is "occupied" by Israel and concluded that irrespective of the sovereign status of Gaza, it is impossible to conclude that it is occupied by Israel.
To your question itself: it appears to stem from imprecise use of the word blockade. This is one of the problems that stems from mistaking political usage of terms for legal ones. No state is required to permit trade across its own borders absent specific treaty obligations to the contrary. Thus, assuming that Gaza has separate sovereign status, Israel has no obligation to permit goods to cross its borders into Gaza. By the same token, Egypt had the right (absent treaty obligations with Israel and other states) to restrict trade crossing Egypt's border into Israel. The Straits of Tiran, however, are international waters. Using force in international waters in order to block shipping reaching another sovereign state, as Egypt did in the Straits of Tiran, is a violation of Israel's sovereignty and a belligerent act. Mislabeling as a boycott Israel's restrictions on trade across its own border with Gaza and the creation a false equivalence with Egypt's siege of Israel in 1967 does not change the legal rules or the relevant facts.
In short: Egypt had the right to control trade across its borders, but not through the international waters of the Straits of Tiran.