Unfortunately that's ambiguous because in England "a loft" and "aloft" would usually sound the same! "The roof was aloft" and "the roof was a loft" have the same pronunciation!

My guess is it was named for the word "aloft" so I would pronounce it "aloft" but the name is stylised as "aloft" and "Aloft" inconsistently across Marriott's own website which makes me wonder, maybe nobody knows? Maybe it's a choose your own adventure type name.

According to the Aloft Hotel franchising document, 6 of the Aloft franchisees have the word "Loft" in their company name which would at least suggest that some franchisees consider the name to be "A Loft" which would make their hotel a "Loft".

The photo on the website of the Aloft Singapore Novena lobby shows a giant letter a printed on the wall and their bar is named "w xyz" (presumably w is a reference to W hotels) which would lend weight to the idea that "a" is a distinct part of the name.

I thought, oh maybe they have adverts or a YouTube channel where the name has been said but their YouTube channel has a single 47 minute video called "Election Noise Escape with Aloft Hotels" which does not once say the brand name (and also raises its own set of questions, like, why?).