Bamboo is treated like dense jungle except as stated otherwise. Dense jungle has airburst so bamboo does as well, but other attacks are also -1 Ordnance/OBA HE, DC, and bombardment.
And it also says : "The Residual FP of an attack that received the bamboo -1 TEM is
increased by one IFT column (like Air Bursts ; A8.26)."
The
like Airburst remark seems to say that the -1 vs HE replaces the Air Burst -1 DRM, even though the rule doesn't explicitely rule out the latter.
I would presume that as a general rule, two different TEM are not cumulative (but Entrenchment and Air Bursts are).
I nevertheless would be astonished that Bamboo suffers -2 vs Indirect Fire, as the rule includes the latter in the broader -1 DRM : the "excepted as stated otherwise" could mean that this broader DRM takes the place of the Air Burst one...