The radiant heat from the fireplace is heating the flooring. My main concern would be rapidly driving any moisture out of the hardwood floor that might cause warpage or cracking.

How hot is the flooring actually getting? At 150F, one can't hold their hand against a metal surface for more than a second or so. Cellulose (wood) self-ignites somewhere in the 400F range, so you're probably nowhere near ignition temps.

You might try a fireplace mat/rug in front to lower the hardwood temps there.

Did he use any Cement board underneath the marble? In my county 1/2 Cement board is REQUIRED. I have FPX 36 " and my hearth is raised (1) foot yet I have 3/4 Cement face board on front near archs of fireplace and on top towards mantle. Then I have 1/2" Cement board under front drawer level with 1/2 Marble cemented with thinset and then hearth front (raised) also has 1/2 cement board plus 1/2 marble cemented to it. When I run my FPX full blast for 3-4 hours the temp there gets like yours get..too hot to touch. (In that narrow area in front of gas fireplace vents & where blower air exits.)
I have friends who have put (1) inch thick marble slab there for the same reason you mention. I may do it too as my flooring is laminate in front of my hearth.[I think these directions were written when people used hardwood floors only.]