Does PIL (3rd edition) repeatedly misuse the length operator on tables and invoke undefined behavior?

[polyglot@...]

5.2 REFERENCE MANUAL

"Unless a __len metamethod is given, the length of a table t is only 
defined
if the table is a sequence, that is, the set of its positive numeric 
keys is
equal to {1..n} for some integer n. In that case, n is its length."

*** Conclusion ***: Without a __len metamethod, an empty table has 
undefined
length.

5.1 REFERENCE MANUAL

"The length of a table t is defined to be any integer index n such that 
t[n]
is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero. 
For a
regular array, with non-nil values from 1 to a given n, its length is
exactly that n, the index of its last value. If the array has "holes" 
(that
is, nil values between other non-nil values), then #t can be any of the
indices that directly precedes a nil value (that is, it may consider any
such nil value as the end of the array)."

*** Conclusion ***: An empty table (t[1] is nil) may or may not have a
length of zero.

===========================================================================

So I'm assuming the following idiomatic code invokes undefined behavior:

a = {}
a[#a + 1] = 'foo'

As does this code (because of the implicit #a+1):

a = {}
table.insert(a, 'foo')

===========================================================================

Yet such code appears repeatedly in PIL (3rd edition), and I haven't
seen any errata for it:

PAGE 94

threads = {}    -- list of all live threads
-- [...]
  table.insert(threads, co)

PAGE 95

local timedout = {}
-- [...]
timedout[#timedout + 1] = res

PAGE 99 and PAGE 100

local words = {}
for w in pairs(counter) do
  words[#words + 1] = w
end

PAGE 113 and PAGE 114

local t = {}
for line in io.lines() do
  t[#t + 1] = -- [...]
end

PAGE 115

path = path or {}
visited = visited or {}
if visited[curr] then   -- node already visited?
  return nil            -- no path here
end
visited[curr] = true    -- mark node as visited
path[#path + 1] = curr  -- add it to path

PAGE 129

function Set.tostring (set)
  local l = {}     -- list to put all elements from the set
  for e in pairs(set) do
    l[#l + 1] = e
  end

PAGE 195

t = {}
for line in io.lines() do
   table.insert(t, line)
end

PAGE 196

a = {}
for n in pairs(lines) do a[#a + 1] = n end

PAGE 197

local a = {}
for n in pairs(t) do a[#a + 1] = n end

PAGE 202

local t = {}                    -- table to store the indices
local i = 0
while true do
  i = string.find(s, "\n", i+1) -- find next newline
  if i == nil then break end
  t[#t + 1] = i
end

PAGE 203

words = {}
for w in string.gmatch(s, "%a+") do
  words[#words + 1] = w
end

PAGE 212

function encode (t)
  local b = {}
  for k,v in pairs(t) do
    b[#b + 1] = (escape(k) .. "=" .. escape(v))
  end

PAGE 218

local a = {}
a[#a + 1] = -- [...]

PAGE 223

local lines = {}
-- read the lines in table 'lines'
for line in io.lines() do lines[#lines + 1] = line end