Foxleys book does not actually claim reason why it need to be preallocated. I've got the book also. Index mentions fsck only page 52, where it reads:
"52 UNIX FOR SUPER-USERS
...
The filestore consistency check is performed by the command fsck (usu-
ally stored as /etc/fsck, but sometimes in /bin), which should be used to check
all discs used as file-systems. It defaults to the list of filestore devices given
in the file /etc/checklist. At this stage, most of the file-systems will not be
mounted, so will be inactive; only the root file-system will be active. The
fsck command goes through each one in turn, reports any inconsistencies in
them, and offers to correct them. The reply to each query is either 'y' for yes
(correct the inconsistency), or 'n' for no (leave the file-system inconsistent).
A parameter '-y' to the command assumes 'yes' replies to all questions, so
that no further interaction is necessary; a parameter '-n' similarly assumes
all 'no' answers, and therefore needs no write permission to the device. Any
'yes' reply may involve the loss of information, such as the complete removal
of a suspect file. Suspect files on the file-system being checked are written to
a directory lost + found on the device if such a directory exists; this directory
must have been created, and be sufficiently large already to hold the names
of all the files involved. This can be ensured by first creating the directory,
then creating a number of files in the directory, and then removing them.
The corrected systems will be consistent, and can later be mounted as and
when required. It may be possible to recover information from deleted files
by looking at the lost + found directory. There should be a lost + found direc-
tory at the head of each mountable file-system.
When checking the root file-system, there are complications, in that it
will be active (even though, because it is root, it will not be formally mounted
as such, but is implicitly mounted as root during the booting process). If
modifications are necessary, they should be completed, and the machine
rebooted without first performing a sync (see section 4.5 below for the nor-
mal procedure for taking a system down). This is to ensure that the disc as
modified by fsck is not overwritten by any in-core information, which may
have been generated from information read from the original corrupt (incon-
sistent) version.
...
"
But I've also read more detailed explanation that recollect is that unless blocks were preallocated, there was a possibility that lost+found need first allocate more blocks directory it already had, it would possibly led to losing some data that would otherwise been able to recover once fsck had advanced further from that point.
Old time UNIX systems directories were just another structured file, which a 'd' bit (like others you changed with chmod) on them, where each record was 16 bytes, which 2 first were the inode number followed by 14 bytes reserved for filename. IIRC linux also had first same limit first filesystems. You could read directory with any program, common feat was to check any odd stuff that "ls" would not show with could hexdump or "od" with some flags and print 16 bytes lines per row. That way you also could see any deleted or moved files from that directory, because directory entry was not quite long otherwise cleared but just clearing that inode reference two bytes.
A Quick look from other books that I have close me now Maurice J. Bach, The Design of Operating UNIX System (-86) contains much more about fsck and filesystem fixing issues scattered few pages in the book and what methods were used to mitigate loss of data. S.R Bourne The Unix System, no mention of fsck at all, at least by looking book automatically generated index.
It could have been some other quite old book I did read, but did not own or anything since BSD4.3-tahoe documentation I've read over the years. But sure it would be nice to read that exact reasoning again from credible sources.
edit: Oh, and you could preallocate also just by adding entries or copying some data to lost+found enough, and then remove entries. Unix traditionally have not compacted and resized directories. They only grow and can be if have been very large slow to traverse. The way to compact is creating another, moving existing data there and then swapping directories.