Say what with certainty?
I can say what their 2017 statement of principles and policies says, with certainty, by reading it. It includes multiple references to Jerusalem:
"Palestine symbolizes the resistance that shall continue until liberation is accomplished, until the return is fulfilled and until a fully sovereign state is established with Jerusalem as its capital."
"Jerusalem is the capital of Palestine. Its religious, historic and civilizational status is fundamental to the Arabs, Muslims and the world at large. Its Islamic and Christian holy places belong exclusively to the Palestinian people and to the Arab and Islamic Ummah. Not one stone of Jerusalem can be surrendered or relinquished. The measures undertaken by the occupiers in Jerusalem, such as Judaization, settlement building, and establishing facts on the ground are fundamentally null and void."
Later (in the same document) qualified by the following:
"Hamas believes that no part of the land of Palestine shall be compromised or conceded, irrespective of the causes, the circumstances and the pressures and no matter how long the occupation lasts. Hamas rejects any alternative to the full and complete liberation of Palestine, from the river to the sea. However, without compromising its rejection of the Zionist entity and without relinquishing any Palestinian rights, Hamas considers the establishment of a fully sovereign and independent Palestinian state, with Jerusalem as its capital along the lines of the 4th of June 1967, with the return of the refugees and the displaced to their homes from which they were expelled, to be a formula of national consensus."
If you're asking how I can be certain about what they mean when they say "Jerusalem" in later statements - I cannot - which is why I used the phrase "I would say that's likelt best understood", however, their rhetoric (even after that 2017 policy statement) usually just says "Jerusalem" as the capital. E.g., when in 2023 Hamas's then-leader stated it was "ready for political negotiations for a two-state solution with Jerusalem as the capital of Palestine".
They have also previously stated (including in 2024) in the same breath that they accept 1967 borders with Jerusalem as the capital of Palestine - an oxymoron on its face, unless you understand "Jerusalem" to mean "East Jerusalem", which isnt what they said.
I think the reasonable reading, as its the only coherent one and is consistent with their stated policy, is that they are using "Jerusalem" to mean East Jerusalem.
If you were to interpret it differently, and say that they mean all of Jerusalem, I dont think that would make sense but it would be worthy of future discussion.
What I dont think it reasonable is to take the position that:
"well, before the UK and France waded in, Hamas's use of the word Jerusalem meant only East Jerusalem, but now that Macron and Starmer have made statements, and Hamas have repeated their previous language, they might suddenly be using the word Jerusalem in a different way (to demand additional territory), even though they haven't given any indication that's actually the case".