Constraints are not the same thing. Poetry locks the entire dependency closure. You can install with “no-deps” option, but then you have to specify specific versions of every dependency in requirements.txt
As an example, let's create a venv and install some older versions of Django and its dependencies (current versions are 0.4.2, 3.5.2 and 4.0.6)
$ python3 -m venv env1
$ ./env1/bin/pip install sqlparse==0.4.0 asgiref==3.5.0 django=4.0.0
$ ./env1/bin/pip install flask
$ ./env1/bin/pip freeze >constraints.txt
$ echo '-c constraints.txt' >requirements.txt
$ echo 'django' >>requirements.txt
$ cat requirements.txt
-c constraints.txt
django
$ cat constraints.txt
asgiref==3.5.0
click==8.1.3
Django==4.0
Flask==2.1.3
itsdangerous==2.1.2
Jinja2==3.1.2
MarkupSafe==2.1.1
sqlparse==0.4.0
Werkzeug==2.1.2
$ python3 -m venv env2
$ ./env2/bin/pip install -r requirements.txt
$ ./env2/bin/pip freeze
asgiref==3.5.0
Django==4.0
sqlparse==0.4.0 poetry init
poetry add x y z
poetry install
poetry remove y