The Legality of “Blockading” Gaza and 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.