To get Flatbuffers to work with Teensy, we need to remove the Arduino specific code for STL (Teensy supports STL directly) in base.h (look for EDIT below, in two places)

base.h

#ifndef FLATBUFFERS_BASE_H_
#define FLATBUFFERS_BASE_H_

// clang-format off

// If activate should be declared and included first.
#if defined(FLATBUFFERS_MEMORY_LEAK_TRACKING) && \
    defined(_MSC_VER) && defined(_DEBUG)
  // The _CRTDBG_MAP_ALLOC inside <crtdbg.h> will replace
  // calloc/free (etc) to its debug version using #define directives.
  #define _CRTDBG_MAP_ALLOC
  #include <stdlib.h>
  #include <crtdbg.h>
  // Replace operator new by trace-enabled version.
  #define DEBUG_NEW new(_NORMAL_BLOCK, __FILE__, __LINE__)
  #define new DEBUG_NEW
#endif

#if !defined(FLATBUFFERS_ASSERT)
#include <assert.h>
#define FLATBUFFERS_ASSERT assert
#elif defined(FLATBUFFERS_ASSERT_INCLUDE)
// Include file with forward declaration
#include FLATBUFFERS_ASSERT_INCLUDE
#endif

#ifndef ARDUINO
#include <cstdint>
#endif

#include <cstddef>
#include <cstdlib>
#include <cstring>

// EDIT: Remove the if defined(ARDUINO) part
#include <utility>

#include <string>
#include <type_traits>
#include <vector>
#include <set>
#include <algorithm>
#include <iterator>
#include <memory>

#ifdef _STLPORT_VERSION
  #define FLATBUFFERS_CPP98_STL
#endif
#ifndef FLATBUFFERS_CPP98_STL
  #include <functional>
#endif

// EDIT: remove the "flatbuffers/" path
#include "stl_emulation.h"

#if defined(__ICCARM__)
#include <intrinsics.h>
#endif